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