Criminal Law
Mens Rea
The mental state or intent required to be guilty of a crime. Literally Latin for 'guilty mind,' it answers the question: what was the defendant thinking when they acted?
Actus Reus
The physical act (or sometimes omission) that constitutes the criminal conduct. Latin for 'guilty act,' it is the external, observable part of a crime — as opposed to mens rea, the mental element.
Double Jeopardy
The constitutional rule that the government cannot try you twice for the same crime after you have been acquitted or convicted. Once the criminal case is over, it's over.
Probable Cause
The legal standard police must meet before making an arrest or conducting a search. It requires more than a hunch — there must be a reasonable basis to believe a crime occurred and the person committed it (for arrests) or that evidence will be found in the place searched (for searches).
Plea Bargain
An agreement between a prosecutor and a defendant in which the defendant pleads guilty in exchange for some benefit — usually reduced charges, a lighter sentence, or both. Over 90% of criminal convictions in the U.S. result from plea bargains, not trials.
Grand Jury
A group of citizens that reviews the prosecution's evidence and decides whether there is enough to formally charge someone with a serious crime. Unlike a trial jury, a grand jury meets in secret and only hears from the prosecution.
Warrant
A court order authorizing law enforcement to conduct a search, make an arrest, or seize property. The Fourth Amendment generally requires police to obtain a warrant before searching homes or making arrests, though there are many exceptions.
Bail
Money or other security deposited with the court to secure a defendant's release from jail while their case is pending. If the defendant shows up for all court dates, the bail is returned; if they flee, it is forfeited.
Parole
Early release from prison under supervision. A parolee serves the rest of their sentence in the community under conditions set by a parole board. Violating those conditions can mean going back to prison.
Probation
A sentence that allows a convicted person to remain in the community instead of going to prison, as long as they comply with court-imposed conditions. It is supervision as an alternative to incarceration.
Felony vs. Misdemeanor
Felonies are serious crimes (like murder, robbery, or rape) typically punishable by more than one year in prison. Misdemeanors are less serious offenses (like petty theft or simple assault) typically punishable by up to one year in jail. The distinction has major consequences beyond just sentence length.
Miranda Rights
The warnings police must give before interrogating someone who is in custody: you have the right to remain silent, anything you say can be used against you, you have the right to an attorney. If police skip these warnings, your statements may be inadmissible at trial.
Constitutional Law
Habeas Corpus
A court order requiring the government to bring a prisoner before a judge and justify why that person is being held. It's a fundamental protection against unlawful imprisonment.
Due Process
The constitutional guarantee that the government cannot deprive you of life, liberty, or property without fair procedures and, in some cases, for substantive reasons. It is the legal requirement that the government play fair.
Equal Protection
The constitutional guarantee that the government must treat similarly situated people alike. Under the Fourteenth Amendment, states cannot discriminate against people without sufficient justification.
Standing
The legal right to bring a lawsuit. You must have standing — a direct, concrete injury caused by the defendant that the court can remedy — before a federal court can hear your case. Without standing, the case is dismissed.
Moot
A case becomes moot when the legal controversy no longer exists — the issue has been resolved, become irrelevant, or can no longer be remedied by a court. Federal courts cannot hear moot cases because there is no live controversy to decide.
Ripeness
A case is not ripe if the legal dispute has not yet fully developed — the harm is still too speculative or hypothetical for a court to decide. Courts cannot issue advisory opinions on future harm that may never occur.
Sovereign Immunity
The doctrine that the government cannot be sued without its consent. Originally derived from the English concept that 'the king can do no wrong,' it now means that federal, state, and sometimes local governments have legal protection from lawsuits unless they waive it.
Eminent Domain
The government's power to take private property for public use, as long as it pays just compensation to the owner. The Fifth Amendment allows the government to take your property even if you don't want to sell — but it must pay you fair market value.
Civil Procedure
Res Judicata
Once a court has issued a final judgment in a case, the same parties cannot litigate the same claims again. It prevents endless re-litigation of the same dispute.
Statute of Limitations
A law that sets the maximum time you have to file a lawsuit or bring criminal charges after an event occurs. If you wait too long, you lose your right to sue or prosecute — forever.
Deposition
A formal interview conducted before trial where a witness answers questions under oath. Depositions are recorded and can be used at trial — either to present a witness's testimony or to impeach them if they change their story.
Discovery
The pretrial process by which parties in a lawsuit can obtain information and evidence from each other and from third parties. Discovery prevents trial by surprise — both sides must reveal what they know.
Summary Judgment
A court ruling that ends a lawsuit before trial because the undisputed facts, viewed in the light most favorable to the non-moving party, show that one side is entitled to win as a matter of law. No triable issue of fact exists.
Injunction
A court order requiring a party to do something or stop doing something. Unlike money damages, an injunction controls future behavior. Violating an injunction is contempt of court.
Jurisdiction
A court's power to hear a particular case. There are two types: subject matter jurisdiction (what kinds of cases the court can hear) and personal jurisdiction (power over the parties). A court without jurisdiction cannot validly decide a case.
Venue
The geographic location where a lawsuit is tried. Even if a court has jurisdiction (power to hear the case), the case might be transferred to a more convenient or appropriate location. Venue is about where, not whether.
Class Action
A lawsuit brought by one or a few plaintiffs on behalf of a large group of people with similar claims. Instead of thousands of individual lawsuits, one case resolves the common issues for everyone in the class.
Torts
Tort
A civil wrong — not a crime — that causes harm to another person and creates legal liability. If you can sue someone for injuring you, the claim is almost certainly based on a tort.
Negligence
Failure to exercise the care that a reasonably careful person would use in similar circumstances, causing harm to another. It is the most common basis for civil lawsuits in the U.S.
Strict Liability
Legal responsibility that attaches regardless of fault or intent. If you engage in certain dangerous activities or sell defective products, you can be held liable even if you took every possible precaution.
Damages
The money awarded to a winning plaintiff in a civil lawsuit to compensate for harm suffered. There are several types: compensatory damages (to make the plaintiff whole), and in some cases punitive damages (to punish the defendant).
Punitive Damages
Extra money damages awarded to punish a defendant for especially egregious conduct and to deter similar behavior in the future. They go beyond compensating the plaintiff — they are about punishment and deterrence.
Evidence
Reasonable Doubt
The highest standard of proof in the legal system, required to convict someone of a crime. The prosecution must prove every element of the charge beyond a reasonable doubt — meaning any reasonable person would be firmly convinced of guilt.
Burden of Proof
The obligation a party has to prove their case to a certain level of certainty. In criminal cases, the prosecution bears this burden and must prove guilt beyond a reasonable doubt. In civil cases, the burden is usually on the plaintiff to prove their case by a preponderance of the evidence.
Court Process
Voir Dire
The process of questioning potential jurors before trial to determine whether they can be fair and impartial. Both sides get to ask questions and remove jurors they believe are biased.
Arraignment
The first formal court appearance after criminal charges are filed. The defendant is read the charges against them and asked to enter a plea of guilty, not guilty, or no contest.
Subpoena
A legal order requiring someone to appear in court to testify or to produce documents. If you receive a subpoena, you must comply — ignoring it is contempt of court.
Appeal
A request to a higher court to review and reverse a lower court's decision. The losing party argues that a legal error was made that affected the outcome. Appeals courts generally do not retry cases — they review whether the law was applied correctly.
Legal Concepts
Stare Decisis
The principle that courts should follow previous decisions when deciding similar cases. It is what makes 'precedent' binding — once a higher court decides an issue, lower courts in the same system must follow that ruling.
Civil vs. Criminal
Criminal cases are brought by the government to punish wrongdoing; civil cases are brought by private parties to resolve disputes and obtain compensation. The key differences are who brings the case, what the burden of proof is, and what the consequences are.
Plaintiff vs. Defendant
In a civil lawsuit, the plaintiff is the person who files the suit and claims to have been harmed; the defendant is the person being sued. In criminal cases, the 'plaintiff' equivalent is the prosecution (the government), and the defendant is the accused.
Precedent
A prior court decision that serves as an example or rule to be followed in future cases involving similar issues. Binding precedent must be followed; persuasive precedent may be considered but is not mandatory.