English and Scottish: from the Middle English, Old French personal name Alain, Alein (Old Breton Alan), from a Celtic personal name of great antiquity and obscurity. In England the personal name is now usually spelled Alan, the surname Allen; in Scotland the surname is more often Allan. From 1139 it was common in Scotland, where the surname also derives from Gaelic Ailéne, Ailín, from ail ‘rock’. The present-day frequency of the surname Allen in England and Ireland is partly accounted for by the popularity of the personal name among Breton followers of William the Conqueror, by whom it was imported first to Britain and then to Ireland. Saint Alan(us) was a 5th-century bishop of Quimper, who was a cult figure in medieval Brittany. Another Saint Al(l)an was a Cornish or Breton saint of the 6th century, to whom a church in Cornwall is dedicated.

English: occasionally perhaps from the rare Middle English femaje personal name Aline (Old French Adaline, Aaline), a pet form of ancient Germanic names in Adal-, especially Adalheidis (see Allis ).

French: variant of Allain , a cognate of 1 above, and, in North America, (also) an altered form of this.