an alternative to something else

someone or something that takes the place of another

स्थानापन्न

English Usage: He is a good substitute for the injured player.

Hindi Usage: वह घायल खिलाड़ी के लिए एक अच्छा स्थानापन्न है।

able to replace something else

परिवर्तनीय

English Usage: She found a substitute teacher for her class.

Hindi Usage: उसने अपनी कक्षा के लिए एक परिवर्तनीय शिक्षक पाया।