n (British|informal) A substitute in a football (soccer) game: someone who comes on in place of another player part way through the game.
n (British|informal|often in plural) Short for subscription: a payment made for membership of a club, etc.
n (computing|programming) A subroutine (sometimes one that does not return a value, as distinguished from a function, which does).
v (British|informal) (football) Less commonly, and often as 'sub on', to bring on (a player) as a substitute.
v (microscopy) To prepare (a slide) with an layer of transparent substance to support and/or fix the sample.