Criminal Law Legal Terms Glossary
Criminal law definitions covering culpability, defenses, and sentencing terms.
Definitions
Mens Rea
Mens rea ("guilty mind") refers to the mental state required for criminal liability. The Model Penal Code identifies four levels of culpability in descending order of blameworthiness: purposely (conscious objective to engage in the conduct or cause the result), knowingly (awareness that conduct is of a certain nature or practically certain to cause the result), recklessly (conscious disregard of a substantial and unjustifiable risk), and negligently (failure to perceive a substantial and unjustifiable risk). The required mens rea varies by offense and must be proven beyond a reasonable doubt.
Actus Reus
Actus reus ("guilty act") is the physical or external component of a crime. It requires a voluntary act or, in limited circumstances, an omission to act when there is a legal duty to do so. Involuntary acts — such as reflexes, convulsions, or conduct during unconsciousness — do not satisfy the actus reus requirement. The actus reus must coincide in time with the mens rea. An omission can constitute actus reus when there is a statutory duty, contractual duty, special relationship, or voluntary assumption of care.
Murder
Murder is the unlawful killing of another human being with malice aforethought. At common law, malice aforethought includes intent to kill, intent to inflict serious bodily harm, depraved-heart murder (extreme recklessness showing indifference to human life), and felony murder (a killing during the commission of a dangerous felony). The Model Penal Code classifies murder as a killing committed purposely or knowingly, or recklessly under circumstances manifesting extreme indifference to the value of human life.
Manslaughter
Manslaughter is the unlawful killing of another human being without malice aforethought. It is divided into voluntary and involuntary manslaughter. Voluntary manslaughter occurs when the defendant kills in the heat of passion upon adequate provocation, which would cause a reasonable person to lose self-control. Involuntary manslaughter involves an unintentional killing caused by criminal negligence or during the commission of an unlawful act not amounting to a felony.
Felony Murder Rule
The felony murder rule imposes murder liability on a defendant who causes a death during the commission or attempted commission of a dangerous felony, regardless of intent to kill. The rule eliminates the need to prove malice aforethought. The inherently dangerous felony limitation requires that the underlying felony be dangerous to human life. Many jurisdictions limit the rule with the merger doctrine (the underlying felony must be independent of the homicidal act) and an agency theory (the killing must be committed by one of the felons).
Self-Defense
Self-defense is a justification defense that permits the use of reasonable force to protect oneself from an imminent threat of unlawful physical harm. The force used must be proportional to the threat: deadly force is permissible only when facing a threat of death or serious bodily harm. In some jurisdictions, the defendant must retreat if safely possible before using deadly force (the duty to retreat), while others follow the 'castle doctrine,' allowing no duty to retreat in one's home. The defendant's belief in the necessity of self-defense must be both honest and reasonable.
Insanity Defense
The insanity defense excuses criminal liability on the ground that the defendant, at the time of the offense, suffered from a severe mental disease or defect that impaired their capacity for rational conduct. The M'Naghten test (the majority rule) requires that the defendant did not know the nature or quality of the act, or did not know it was wrong. The Model Penal Code test is broader, excusing a defendant who lacked substantial capacity to appreciate the criminality of their conduct or to conform their conduct to the law.
Entrapment
Entrapment is a defense asserting that the government induced the defendant to commit a crime that the defendant was not predisposed to commit. Under the subjective test (the majority approach), the focus is on the defendant's predisposition — a predisposed defendant cannot claim entrapment regardless of government inducement. Under the objective test (the minority/MPC approach), the focus is on the government's conduct — whether it would likely induce a law-abiding person to commit the crime. Entrapment applies only to government agents, not private individuals.
Conspiracy
Conspiracy is an agreement between two or more persons to commit an unlawful act. At common law, the agreement itself is the actus reus. Many jurisdictions and the Model Penal Code also require an overt act in furtherance of the conspiracy. Each conspirator is liable for all foreseeable crimes committed by co-conspirators in furtherance of the conspiracy (Pinkerton liability). The MPC follows a unilateral approach (one party's genuine agreement suffices), while common law requires a bilateral agreement.
Attempt
Attempt is an inchoate crime requiring the specific intent to commit the target offense and a substantial step toward its completion that strongly corroborates criminal purpose. Mere preparation is insufficient — the defendant must go beyond planning and take action that demonstrates a firm resolution to commit the crime. Under the Model Penal Code, any act constituting a substantial step that is strongly corroborative of criminal purpose suffices. Factual impossibility is generally not a defense, while legal impossibility may be.
Accomplice Liability
Accomplice liability holds a person criminally responsible for another's crime when they intentionally assist, encourage, or facilitate its commission. Under both common law and the Model Penal Code, the accomplice must act with the purpose of promoting or facilitating the offense. An accomplice is liable for the principal's crime as well as any other crimes that are the natural and probable consequences of the intended crime (in jurisdictions following that doctrine). Mere presence at the scene, without more, is insufficient.
Solicitation
Solicitation is the inchoate crime of asking, encouraging, or requesting another person to commit a crime, with the intent that the crime be committed. The crime is complete upon the asking — it does not matter whether the person solicited agrees, attempts, or completes the crime. Under the Model Penal Code, solicitation merges into the completed crime or attempt if the solicited person acts. Some jurisdictions treat solicitation as a lesser offense than the target crime.
Double Jeopardy
The Double Jeopardy Clause of the Fifth Amendment protects individuals from being tried or punished twice for the same offense after acquittal, after conviction, or against multiple punishments for the same offense. Jeopardy 'attaches' in a jury trial when the jury is sworn and in a bench trial when the first witness is sworn. The dual sovereignty doctrine allows separate state and federal prosecutions for the same conduct because they are different sovereigns. Double jeopardy does not apply to civil proceedings.
Habeas Corpus
Habeas corpus ("you shall have the body") is a legal procedure through which a prisoner can challenge the legality of their detention. It is enshrined in Article I, Section 9 of the Constitution, which limits Congress's power to suspend the writ. Federal habeas corpus review under 28 U.S.C. Section 2254 allows state prisoners to challenge their convictions in federal court on the grounds that their detention violates the Constitution. AEDPA (1996) significantly restricted the scope of federal habeas review.
Miranda Rights
Miranda rights are the constitutional warnings that law enforcement must give to individuals in custodial interrogation, as required by Miranda v. Arizona. An officer must inform the suspect of their right to remain silent, that anything they say can be used against them, their right to an attorney, and that an attorney will be provided if they cannot afford one. Statements obtained without Miranda warnings are inadmissible in the prosecution's case-in-chief, though they may be used for impeachment.
Probable Cause
Probable cause is the Fourth Amendment standard required for an arrest or the issuance of a search warrant. It exists when the facts and circumstances within an officer's knowledge are sufficient to warrant a person of reasonable caution in the belief that a crime has been, is being, or is about to be committed (for arrest), or that evidence of a crime will be found in the place to be searched (for a warrant). Probable cause is a practical, non-technical standard — more than reasonable suspicion but less than proof beyond a reasonable doubt.
Reasonable Suspicion
Reasonable suspicion is a standard lower than probable cause, requiring specific and articulable facts that, taken together with rational inferences, justify a brief investigatory stop (a Terry stop). It is more than a mere hunch but less than probable cause. The officer must be able to point to objective facts supporting the suspicion of criminal activity. The stop must be temporary and limited in scope to the purpose of confirming or dispelling the officer's suspicion.
Exclusionary Rule
The exclusionary rule prohibits the use of evidence obtained through unconstitutional government conduct — typically an illegal search or seizure in violation of the Fourth Amendment — in a criminal prosecution. The rule serves as a deterrent against police misconduct. Exceptions include the good-faith exception (when officers reasonably rely on a facially valid warrant), the inevitable discovery doctrine, the independent source doctrine, and attenuation of the taint.
Fruit of the Poisonous Tree
The fruit of the poisonous tree doctrine extends the exclusionary rule to exclude not only illegally obtained evidence (the 'poisonous tree') but also any evidence derived from or discovered as a result of the initial illegality (the 'fruit'). This prevents the government from benefiting indirectly from unconstitutional conduct. The doctrine has three recognized exceptions: the independent source doctrine, the inevitable discovery doctrine, and the attenuation doctrine, which breaks the causal chain between the illegality and the evidence.
Beyond a Reasonable Doubt
Beyond a reasonable doubt is the highest standard of proof in the American legal system, required for a criminal conviction. The prosecution must prove every element of the offense to such a degree that there is no reasonable doubt in the mind of a rational juror. This standard is constitutionally required under the Due Process Clause, as established in In re Winship. Reasonable doubt is not any possible doubt or speculative doubt — it is doubt based on reason and common sense after careful consideration of the evidence.