substitute

a person or thing that takes or can take the place of another

an athlete who plays only when a starter on the team is replaced

someone who takes the place of another (as when things get dangerous or difficult); "the star had a stand-in for dangerous scenes"; "we need extra employees for summer fill-ins"

put in the place of another; switch seemingly equivalent items; "the con artist replaced the original with a fake Rembrandt"; "substitute regular milk with fat-free milk"; "synonyms can be interchanged without a changing the context's meaning"

be a substitute; "The young teacher had to substitute for the sick colleague"; "The skim milk substitutes for cream--we are on a strict diet"

act as a substitute; "She stood in for the soprano who suffered from a cold"

capable of substituting in any of several positions on a team; "a utility infielder"

serving or used in place of another; "an alternative plan"

artificial and inferior; "ersatz coffee"; "substitute coffee"