substitute

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.
3. One who enlists for military service in the place of a conscript.

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; to replace X with Y.

   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).

noun

A good that a consumer perceives as similar to another good, which decreases the demand for that other good