Legal Latin terms are the compact, centuries-old phrases that still run through American contracts, courtrooms, and statutes because each one carries a precise meaning that plain English tends to blur. The glossary below walks through the ones you are most likely to encounter, grouped by where they show up, with the practical consequence of each.
Criminal Law
Mens Rea
Mens rea translates roughly to “guilty mind.” It refers to the mental state the prosecution must prove the defendant had when committing the crime, and the required level of intent varies by offense. Murder requires showing the defendant intended to kill. A lesser charge like reckless endangerment only requires showing the defendant consciously disregarded a known risk. Without some form of culpable mental state, a harmful act is generally treated as an accident rather than a crime.1Legal Information Institute. Mens Rea
Actus Reus
Actus reus is the other half of the equation: the physical act of committing the crime. Thinking about robbing a bank is not a crime. Walking in with a weapon and demanding money is. Prosecutors must prove both the guilty act and the guilty mind beyond a reasonable doubt, and failing on either element means an acquittal. The pairing exists to prevent the government from punishing people for thoughts alone or for causing harm by genuine accident.1Legal Information Institute. Mens Rea
Nolo Contendere
Nolo contendere means “I do not wish to contend.” A defendant who enters this plea accepts the criminal penalty without admitting guilt. The court treats it like a guilty plea for sentencing, but with one significant difference: the plea cannot be used against the defendant as an admission of guilt in a later civil lawsuit.2Legal Information Institute. Nolo Contendere If someone facing a drunk-driving charge knows a personal-injury lawsuit is coming next, this plea avoids handing the plaintiff a ready-made admission to use in civil court. Judges are not required to accept the plea and sometimes reject it, particularly in serious cases where the public interest favors a clear admission.
Negligence and Liability
Res Ipsa Loquitur
Res ipsa loquitur means “the thing speaks for itself.” When an injury is the kind that simply does not happen without negligence, a plaintiff can use this doctrine instead of producing direct evidence of what went wrong. The classic example is a barrel that rolls out of a second-story warehouse window and lands on a pedestrian. Barrels do not fall from buildings when everyone is being careful.3Legal Information Institute. Res Ipsa Loquitur
To invoke the rule, a plaintiff generally must show that the harm would not ordinarily occur without negligence, that the thing causing the injury was under the defendant’s control, and that no other plausible explanation exists. Once those conditions are met, the burden shifts to the defendant to explain what happened. The doctrine creates a rebuttable presumption rather than automatic liability.3Legal Information Institute. Res Ipsa Loquitur
Negligence Per Se
Negligence per se means “negligence in itself.” When a defendant violates a safety statute and that violation causes exactly the kind of harm the statute was designed to prevent, courts treat the violation as automatic proof of negligence. The plaintiff does not have to prove the defendant breached a duty of care, because the statutory violation establishes the breach on its own.4Legal Information Institute. Per Se
A driver who runs a red light and hits a pedestrian illustrates the concept well. Traffic lights exist to prevent collisions, pedestrians are among the people the law protects, and the collision is the type of harm the law targets. The plaintiff still has to connect the violation to the injury. If a driver ran a red light three blocks before an accident caused by a tire blowout, the statutory violation did not cause the crash, and negligence per se would not apply.
Prima Facie
Prima facie means “at first sight.” A prima facie case is one where the plaintiff has presented enough evidence to support the claim on its face, creating a rebuttable presumption that shifts the burden to the other side. If the defendant fails to counter that evidence, the plaintiff wins.5Legal Information Institute. Prima Facie
You hear the phrase most often in employment discrimination claims, where a plaintiff establishes a prima facie case by showing they belong to a protected class, were qualified for the position, suffered an adverse action, and can point to circumstances suggesting discrimination. The employer then gets a chance to offer a legitimate, non-discriminatory explanation. The term is not limited to any single area of law and appears wherever courts evaluate whether enough initial evidence exists to let a claim move forward.
Contracts, Property, and Business
Quid Pro Quo
Quid pro quo means “something for something.” In contract law, it captures the idea that enforceable agreements require each side to give up something of value. That mutual exchange of value is called “consideration,” and without it, most promises are unenforceable. Promise to give a friend your old car and expect nothing in return, and that arrangement is a gift, not a contract. A court will not order you to follow through.
Outside contracts, the phrase appears in employment and corruption cases. A supervisor who demands sexual favors in exchange for a promotion engages in quid pro quo harassment. A public official who trades political favors for campaign donations may face corruption charges built around the same concept. One benefit exchanged for another.
Bona Fide
Bona fide means “in good faith.” Parties to a contract are expected to deal honestly, disclose material information, and follow through on their commitments. When someone hides a known defect in a product or lies about the terms of a deal, they have violated this standard, and the other party may be entitled to void the agreement or recover damages.
Caveat Emptor
Caveat emptor means “let the buyer beware.” Under this doctrine, the buyer bears responsibility for inspecting what they are purchasing and discovering defects before closing. Skip the inspection and find a cracked foundation six months later, and you may have no legal remedy. Modern consumer protection laws have narrowed the doctrine considerably for retail purchases, but it retains real force in many real estate transactions and private sales of used goods. The doctrine does have limits: a seller who actively conceals a defect or lies about the property’s condition cannot hide behind it.6Legal Information Institute. Caveat Emptor
Force Majeure
Force majeure means “superior force.” The clause appears in many contracts and excuses one or both parties from performing their obligations when extraordinary, unforeseeable events make performance impossible. Qualifying events typically include natural disasters, wars, and major disruptions like pandemics. Courts have recognized COVID-19 shutdowns as force majeure events in some cases.7Legal Information Institute. Force Majeure
The bar is high. A general economic downturn or a deal that turns out to be more expensive than expected does not qualify. Courts look at whether the specific event was listed in the contract’s force majeure clause, and some jurisdictions interpret these clauses narrowly, granting relief only when the exact type of disruption is explicitly named. If your contract says “natural disasters” and a labor strike shuts down your supplier, you might be out of luck.
Lis Pendens
Lis pendens means “pending litigation.” It is a notice filed in public records warning anyone interested in a property that a lawsuit affecting the title is underway. The filing creates what is called a “cloud on title,” which effectively freezes most real estate transactions. A property owner can still technically sell the land, but any buyer takes the property subject to the outcome of the pending lawsuit.8Legal Information Institute. Notice of Pendency
In practice, a lis pendens blocks sales because title companies usually refuse to issue insurance on a clouded title, and most buyers will not close without title insurance. A buyer who proceeds despite the notice cannot claim the protections of a bona fide purchaser and inherits whatever liability the lawsuit produces.
Courtroom Procedure
Subpoena
A subpoena, from the Latin “under penalty,” is a court order requiring a person to appear and testify, produce documents, or both. Ignoring one is not optional. A person who disregards a properly served subpoena can be held in contempt of court, which carries fines and potentially jail time until they comply.9Legal Information Institute. Subpena (Subpoena)
You will sometimes see a distinction between a subpoena ad testificandum, which compels a person to appear and give testimony, and a subpoena duces tecum, which compels the production of specific documents or records like financial statements, emails, or medical files. Both carry the same penalties for non-compliance.
Ex Parte
Ex parte means “from one party.” An ex parte proceeding is one where a court hears from only one side, usually because the situation is so urgent that waiting to notify the other party would cause irreparable harm. A common example is a temporary restraining order to freeze assets that might otherwise be hidden before the opposing party can be heard.10Legal Information Institute. Ex Parte
Courts treat ex parte relief as an extraordinary measure. Under federal rules, the party seeking the order must show that immediate, irreparable injury will result before the other side can respond, and must explain in writing what efforts were made to provide notice. A temporary restraining order issued ex parte is short-lived, typically lasting no more than 14 days, and the court schedules a full hearing at the earliest possible time so the restrained party gets a chance to respond.
Stare Decisis
Stare decisis means “let the decision stand.” The doctrine requires courts to follow the rulings of higher courts in the same jurisdiction when the facts and legal issues are similar. A state trial court is bound by its state supreme court’s decisions, and all federal courts are bound by Supreme Court rulings on federal issues.11Legal Information Institute. Precedent If you know how a court ruled on a set of facts last year, you can reasonably expect it to rule the same way on similar facts this year. Courts do overturn precedent occasionally, but they need compelling reasons.
De Novo
De novo means “anew” or “from the beginning.” When an appellate court conducts a de novo review, it examines the legal questions in a case without deferring to the lower court’s conclusions. The appellate court may look at the same evidence but reaches its own independent judgment on what the law requires.12Legal Information Institute. De Novo De novo review of legal questions on appeal is quite common. A full de novo retrial of facts is rare because of the judicial resources involved.
Writ of Certiorari
A writ of certiorari is an order from a higher court directing a lower court to send up the record of a case for review. This is the primary way the U.S. Supreme Court selects which cases to hear. The Court receives thousands of petitions each year and grants certiorari in fewer than 100, so a denial simply means the lower court’s ruling stands.13United States Courts. Supreme Court Procedures
Amicus Curiae
Once a court takes a case, outside parties sometimes weigh in through an amicus curiae brief. The phrase means “friend of the court,” and these briefs bring information or perspectives that the parties themselves might not raise. An amicus might be a trade group explaining how a ruling could affect an industry, a civil rights organization arguing about constitutional implications, or a group of academic experts providing technical context. Courts welcome filings that contain relevant information not already presented but discourage those that merely repeat the parties’ arguments.14Legal Information Institute. Supreme Court Rule 37 – Brief for an Amicus Curiae
Representation, Status, and Rights
Pro Se
Pro se means “for oneself.” A person who handles their own case in court without hiring a lawyer is appearing pro se. Federal law explicitly protects this right, allowing parties to “plead and conduct their own cases personally” in all federal courts.15Office of the Law Revision Counsel. 28 USC 1654 The catch is that pro se litigants are held to the same rules of law and evidence as attorneys. A judge will not relax filing deadlines or procedural requirements because you do not have a law degree. Going pro se saves money on legal fees, which commonly run several hundred dollars per hour, but it carries real risk of procedural mistakes that can sink an otherwise strong case.
Pro Bono
Pro bono is short for pro bono publico, meaning “for the public good.” It refers to legal work performed without charge, usually for people who cannot afford an attorney. The American Bar Association recommends that every lawyer provide at least 50 hours of pro bono service per year, and nearly every state has an ethical rule encouraging the practice.16American Bar Association. ABA Model Rule 6.1 If you cannot afford a lawyer, pro bono programs run by bar associations and legal aid organizations are worth investigating before deciding to go pro se.
In Loco Parentis
In loco parentis means “in the place of a parent.” When a child is enrolled in school or placed with an institution, that entity takes on some of the responsibilities that normally belong to parents, including authority over discipline and safety decisions. The doctrine is a limited delegation. The school acts in place of the parent while the child is under its direct supervision, not permanently or in all matters.
Guardian Ad Litem
A guardian ad litem fills a different role. The term means “guardian for the suit,” and it refers to a person, usually a lawyer, appointed by a court to represent a child’s best interests during a specific legal proceeding like a custody dispute.17Legal Information Institute. Ad Litem Unlike a general guardian who manages all of a child’s affairs, a guardian ad litem’s authority lasts only for the duration of the case. They conduct independent investigations, review records from schools, doctors, and social service agencies, and submit recommendations to the judge. Their job is to speak for the child when the child’s interests might differ from what either parent wants.
Habeas Corpus
Habeas corpus means “you have the body.” A writ of habeas corpus orders the government to bring a detained person before a court and justify the detention. If the court finds no legal basis for holding the person, it can order their release.18Legal Information Institute. Habeas Corpus
The writ has deep roots. The Constitution specifically limits the government’s power to suspend it, permitting suspension only during rebellion or invasion when public safety demands it. In practice, habeas corpus petitions come up most often when prisoners challenge the legality of their conviction or sentence, when immigration detainees contest prolonged detention, or when someone argues their constitutional rights were violated during the criminal process. It remains the primary mechanism for ensuring the executive branch cannot hold people indefinitely without judicial oversight.19United States Courts. Habeas Corpus