Substitute
Part of speech: noun
Definition: A replacement or stand-in for something that achieves a similar result or purpose. A player who is available to replace another if the need arises, and who may or may not actually do so.
Part of speech: verb
Definition: To use in place of something else, with the same function. In the phrase "substitute X for Y", to use X in place of Y. In the phrase "substitute X with/by Y", to use Y in place of X. To remove (a player) from the field of play and bring on another in his place. To serve as a replacement (for someone or something)
Example sentence: Be brave. Take risks. Nothing can substitute experience.
Replacement
Part of speech: noun
Definition: A person or thing that takes the place of another; a substitute.
Example sentence: The thought of going through a bone marrow transplant, which in my case called for a life-threatening dose of chemotherapy followed by a total replacement of my body's bone marrow, was scary enough. But then I learned that finding a donor can be the scariest part of all.