Lycanthropy is the ability or power of a human being to undergo transformation into a wolf, or to gain wolf-like characteristics. The term comes from Greek lykánthropos (λυκάνθρωπος): λύκος, lýkos ("wolf") + άνθρωπος, ánthrōpos ("human"). The word lycanthropy is sometimes used generically for any transformation of a human into animal form, though the precise term for that is technically "therianthropy". Sometimes, "zoanthropy" is used instead of "therianthropy". The word has also been linked to Lycaon, a king of Arcadia who, according to Ovid's Metamorphoses, was turned into a ravenous wolf in retribution for attempting to serve human flesh (his own son) to visiting Zeus in an attempt to disprove the god's divinity.

There is also a mental illness called lycanthropy in which a patient believes he or she is, or has transformed into, an animal and behaves accordingly. This is sometimes referred to as clinical lycanthropy to distinguish it from its use in legends.