Reaction inhibitor

A reaction inhibitor is a substance that decreases the rate of, or prevents, a chemical reaction.

Inhibition of a catalyst
An inhibitor can reduce the effectiveness of a catalyst in a catalysed reaction (either a non-biological catalyst or an enzyme). E.g., if a compound, which similar to (one of) the reactants, but which cannot undergo a reaction that the catalyst can facilitate, enters the active site of a catalyst, the catalyst cannot perform its job. When the inhibitor releases, the catalyst is again available for reaction.

Inhibition and catalyst poisoning
Inhibition should be distinguished from catalyst poisoning, in the former case an inhibitor only hinders the working of a catalyst, while in the latter case the catalyst is destroyed by an irreversible reaction with a chemical substance (the active catalyst in the latter case may be regained by a separate process).