Promotion of a pawn having reached the eighth rank to a piece not being a queen. The most common minor promotion, if happening, is the promotion to a knight (often with check) in order to continue a combination. This type of promotion is sometimes also designated as 'underpromotion'.
A well-known minor promotion occurred in Korody-Balogh, Budapest 1933: 1.d4 d5 2.c4 e5 3.dxe5 d4 4.e3 Bb4+ 5.Bd2 dxe3! 6.Bxb4 exf2+ 7.Ke2 fxg1N+ 8.Rxg1 Bg4+ and White resigned.