A Lay (Norman French Lai) is a short narrative or lyrical poem written in octosyllabic couplets and it deals with tales of adventure, romance, and chivalry. These were believed to have been based on Celtic legends. Helen Cooper called the genre the “mini-Romance” since the typical theme and content deals with courtly love and the other concerns of medieval romance.
The earliest lay narratives were written in the 12th century by Marie De France (c. 1175) whose collection of Lais (c. 1155-70) were twelve Celtic tales of romance that often involved elements of the fantastic.
The term ‘Breton lay’ was applied to 14th c. English poems with a Breton setting and similar to those by Marie de France.
A dozen or more are extant in English, the best known being Sir Orfeo, Havelok the Dane, Sir Launfal, and Chaucer’s Franklin’s Tale. Since the 16th c., the term lay has been used more loosely to denote any historical ballad or narrative of adventure. Good examples are Walter Scott’s Lay of the Last Minstrel (1805) and Macaulay’s Lays of Ancient Rome(1842).
Also read; A short note on Idyll: Definition, Characteristics, and Examples