Here's what GroveMusic has as a definition:
Ballad
(from Lat. ballare: 'to dance').
Term used for a short popular song that may contain a narrative element. Scholars take it to signify a relatively concise composition known throughout Europe since the late Middle Ages: it combines narrative, dramatic dialogue and lyrical passages in stanzaic form sung to a rounded tune, and often includes a recurrent refrain. Originally the word referred to dance-songs such as the carole, but by the 14th century it had lost that connotation in English and had become a distinctive song type with a narrative core. The word has sometimes been used, mistakenly, as a translation for the medieval French forme fixe ballade, and for the 18th- and 19th-century German ballade; the latter was partly influenced by the narrative strophic folksong tradition of Britain and Scandinavia.

The 'ballad opera', a satirical form of theatrical entertainment based on spoken dialogue and popular tunes of the day, was fashionable for several decades during the early to mid-18th century. Literary ballads which imitated the traditional ballad marked a significant phase of influence during the Romantic period. In the 19th century 'ballad' came to denote a sentimental song cultivated by the middle classes in Britain and North America, while in 20th-century popular culture it has come to refer to a slow, personalized love song or one, such as the 'blues ballad' in North America, in which the narrative element is slender and subordinated to a lyrical mood.

Porter, James: 'Ballad', The New Grove Dictionary of Music Online ed. L. Macy (Accessed 11 Mar. 2003),