A body of rules and statutes that defines conduct prohibited by
the government because it threatens and harms public safety and welfare and that establishes punishment to be imposed for the commission of such acts.
The term criminal law generally refers to substantive criminal laws. Substantive criminal laws define crimes and may establish punishments. In contrast, Criminal Procedure describes the process through which the criminal laws are enforced. For example, the law prohibiting murder is a substantive criminal law. The manner in which government enforces this substantive law—through the gathering of evidence and prosecution—is generally considered a procedural matter.
Crimes are usually categorized as felonies or misdemeanors based on their nature and the maximum punishment that can be imposed. A felony involves serious misconduct that is punishable by death or by imprisonment for more than one year. Most state criminal laws subdivide felonies into different classes with varying degrees of punishment. Crimes that do not amount to felonies are misdemeanors or violations. A misdemeanor is misconduct for which the law prescribes punishment of no more than one year in prison. Lesser offenses, such as traffic and parking infractions, are often called violations and are considered a part of criminal law.
The power to make certain conduct illegal is granted to Congress by virtue of the Necessary and Proper Clause of the Constitution (art. I, § 8, cl. 18). Congress has the power to define and punish crimes whenever it is necessary and proper to do so, in order to accomplish and safeguard the goals of government and of society in general. Congress has wide discretion in classifying crimes as felonies or misdemeanors, and it may revise the classification of crimes.
State legislatures have the exclusive and inherent power to pass a law prohibiting and punishing any act, provided that the law does not contravene the provisions of the U.S. or state constitution. When classifying conduct as criminal, state legislatures must ensure that the classification bears some reasonable relation to the welfare and safety of society. Municipalities may make designated behavior illegal insofar as the power to do so has been delegated to them by the state legislature.
Laws passed by Congress or a state must define crimes with certainty. A citizen and the courts must have a clear understanding of a criminal law's requirements and prohibitions. The elements of a criminal law must be stated explicitly, and the statute must embody some reasonably discoverable standards of guilt. If the language of a statute does not plainly show what the legislature intended to prohibit and punish, the statute may be declared void for vagueness.
In deciding whether a statute is sufficiently certain and plain, the court must evaluate it from the standpoint of a person of ordinary intelligence who might be subject to its terms. A statute that fails to give such a person fair notice that the particular conduct is forbidden is indefinite and therefore void. Courts will not hold a person criminally responsible for conduct that could not reasonably be understood to be illegal. However, mere difficulty in understanding the meaning of the words used, or the Ambiguity of certain language, will not nullify a statute for vagueness.
