In ordinary language, a crime is an unlawful act punishable by a state or other authority. The term crime does not, in modern criminal law, have any simple and universally accepted definition, though statutory definitions have been provided for certain purposes. The most popular view is that crime is a category created by law; in other words, something is a crime if declared as such by the relevant and applicable law. One proposed definition is that a crime or offence (or criminal offence) is an act harmful not only to some individual but also to a community, society, or the state ("a public wrong"). Such acts are forbidden and punishable by law.

The notion that acts such as murder, rape, and theft are to be prohibited exists worldwide. What precisely is a criminal offence is defined by the criminal law of each relevant jurisdiction. While many have a catalogue of crimes called the criminal code, in some common law nations no such comprehensive statute exists.

The state (government) has the power to severely restrict one's liberty for committing a crime. In modern societies, there are procedures to which investigations and trials must adhere. If found guilty, an offender may be sentenced to a form of reparation such as a community sentence, or, depending on the nature of their offence, to undergo imprisonment, life imprisonment or, in some jurisdictions, death.

Usually, to be classified as a crime, the "act of doing something criminal" (actus reus) must – with certain exceptions – be accompanied by the "intention to do something criminal" (mens rea).

While every crime violates the law, not every violation of the law counts as a crime. Breaches of private law (torts and breaches of contract) are not automatically punished by the state, but can be enforced through civil procedure.