Meaning

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

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

noun someone who takes the place of another (as when things get dangerous or difficult)

originFrom Middle English substituten, from Latin substitutus, past participle of substituō, from sub- (“under; beneath”) + statuō (“to put up; establish”).

Etymology from Wiktionary, CC BY-SA 3.0