In folklore, lycanthropy is the ability or power of a human being to undergo transformation into a wolf. The term comes from Greek lykánthropos (λυκάνθρωπος): λύκος, lýkos ("wolf") + άνθρωπος, ánthrōpos ("man") (Rose, 230). The word can also be used transitively, referring to the act of transforming someone else into a wolf, or werewolf.
The word lycanthropy is often 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" (Guiley, 192).
Folk-etymology also links the word 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.