reactant

noun

1. a person or thing that reacts.
2. Also called interactant. Chemistry. any substance that undergoes a chemical change in a given reaction.

noun

1. a substance that participates in a chemical reaction, esp a substance that is present at the start of the reactionCompare product (def. 4)

1901 (n.), 1911 (adj.), from react + -ant.

n.

1. A substance participating in a chemical reaction, especially a directly reacting substance present at the start of the reaction.

1. A substance participating in a chemical reaction, especially one present at the start of the reaction.