Concurrence refers to the requirement in criminal law that the mens rea (guilty mind) must coincide with the actus reus (guilty act) for a defendant to be held liable for a crime. This means that at the time of committing the unlawful act, the individual must have the necessary mental state to match their actions. This concept ensures that individuals are not convicted of crimes unless both their intention and actions align, reinforcing the principle of fairness in legal accountability.