A substitute, often in sports or teaching.
Subsistence money: part of a worker's wages paid before the work is finished.
A subtitle.
A submarine sandwich: a sandwich made on a long bun.
A subroutine (sometimes one that does not return a value, as distinguished from a function, which does).
A subcontractor.
A subeditor.
A submarine.
A subwoofer.
A subscription: a payment made for membership of a club, etc.
A submissive.
A subscription (or (by extension) a subscriber) to an online channel or feed.
To replace (a player) with a substitute.
To substitute for.
To lend.
To subscribe.
To take a submissive role.
To coat with a layer of adhering material; to planarize by means of such a coating.
To work as a substitute teacher, especially in primary and secondary education.
To prepare (a slide) with a layer of transparent substance to support and/or fix the sample.
Less commonly, and often as sub on, to bring on (a player) as a substitute.
To subtitle (usually a film or television program).
To perform the work of a subeditor or copy editor; to subedit.
Under.
A particular proposition, as opposed to a universal one.
Succeeding by turns; successive.
Subaltern; inferior; subordinate.