Where, in a piece
of music in a minor key
, the chord
at the end of a perfect cadence
. Most common in pieces composed before the mid-eighteenth century
It is also known, though less frequently, as a Picardie third- the word 'tierce' means 'third' and refers to the way that the third note of the chord is raised to make it major.
An example can be found at the end of the Coventry carol (15th Century Renaissance, words and music: traditional), recently brought to fame in Britain by an NSPCA advert which features the song.