Where, in a
piece or
section of music in a
minor key, the
chord at the end of a
perfect cadence becomes
major. 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.