Which element specifies that a crime must be defined by law?

The correct answer is the principle of legality. This element is fundamental in criminal law and asserts that for any act to be classified as a crime, it must be clearly defined and prohibited by law prior to its commission. This principle ensures that individuals have the right to know what behaviors are considered criminal, thereby providing a safeguard against arbitrary enforcement of the law.

Under the legality principle, no one can be punished for an act that was not defined as a crime at the time it was committed, which upholds the notion of legal certainty and protects individuals from ex post facto laws, where people would be punished for actions that were not crimes at the time they occurred.

Understanding this principle is key for law enforcement and corrections professionals as it underpins the entire criminal justice system, ensuring that laws are applied fairly and justly.