Verb

1. To use in place of something else, with the same function.

   I had no shallots so I substituted onion.

2. In the phrase "substitute X for Y", to use X in place of Y.

   I had to substitute new parts for the old ones.

3. In the phrase "substitute X with/by Y", to use Y in place of X.

   I had to substitute old parts with the new ones.

4. To remove (a player) from the field of play and bring on another in his place.

   He was playing poorly and was substituted after twenty minutes

5. To serve as a replacement (for someone or something)
   • 1987, James Tobin, Essays in Economics, Vol. 2, p. 75

     Accumulation of wealth by this route may substitute for personal saving.

Usage notes

The verb "to substitute" can be used transitively in two opposite ways. "To substitute X" may mean either "use X in place of something else" (as in definitions 1 and 2), or "use something else in place of X" (as in definitions 3 and 4). The latter use has been considered incorrect by some, but it is widespread and now generally accepted (see COED note).

Noun

1. A replacement or stand-in for something that achieves a similar result or purpose.
2. A player who is available to replace another if the need arises, and who may or may not actually do so.