The Manx name of the Isle of Man is Ellan Vannin: ellan (Manx pronunciation: [ɛlʲən]) is a Manx word meaning 'island'; Mannin (IPA: [manɪn]) appears in the genitive case as Vannin (IPA: [vanɪn]), with initial consonant mutation, hence Ellan Vannin, "Island of Mann". The short form often used in English, Mann, is derived from the Manx Mannin, though sometimes the name is written as Man. The earliest recorded Manx form of the name is Manu or Mana.
The Old Irish form of the name is Manau or Mano. Old Welsh records named it as Manaw, also reflected in Manaw Gododdin, the name for an ancient district in north Britain along the lower Firth of Forth. The oldest known reference to the island calls it Mona, in Latin (Julius Caesar, 54 BC); in the 1st century, Pliny the Elder records it as Monapia or Monabia, and Ptolemy (2nd century) as Monœda (Mοναοιδα, Monaoida) or Mοναρινα (Monarina), in Koine Greek. Later Latin references have Mevania or Mænavia (Orosius, 416), and Eubonia or Eumonia by Irish writers. It is found in the Sagas of Icelanders as Mön.
The name is probably cognate with the Welsh name of the island of Anglesey, Ynys Môn, usually derived from a Celtic word for 'mountain' (reflected in Welsh mynydd, Breton menez, and Scottish Gaelic monadh), from a Proto-Celtic *moniyos.
The name was at least secondarily associated with that of Manannán mac Lir in Irish mythology (corresponding to Welsh Manawydan fab Llŷr). In the earliest Irish mythological texts, Manannán is a king of the otherworld, but the 9th-century Sanas Cormaic identifies a euhemerised Manannán as "a famous merchant who resided in, and gave name to, the Isle of Man". Later, a Manannán is recorded as the first king of Mann in a Manx poem (dated 1504).