1. verb
a) To use in place of something else, with the same function.
I had no shallots so I substituted onion.
b) In the phrase "substitute X for Y", to use X in place of Y.
I had to substitute new parts for the old ones.
2. noun
a) A replacement or stand-in for something that achieves a similar result or purpose.
b) A player who is available to replace another if the need arises, and who may or may not actually do so.
Wikipedia foundation.