substitute

artificial and inferior

ersatz coffee

substitute coffee

serving or used in place of another

an alternative plan

capable of substituting in any of several positions on a team

a utility infielder

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

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

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

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

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