substitute - noun

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)

substitute - verb

• put in the place of another; switch seemingly equivalent items
• be a substitute
• act as a substitute

substitute - adjective

• capable of substituting in any of several positions on a team
• serving or used in place of another
• artificial and inferior

substitute - thesaurus

alternate   alternative   backup   deputise   deputize   ersatz   exchange   interchange   relief   reliever   replace   replacement   reserve   sub   utility