Final answer:
The term for the reactant that combines with an enzyme in a reaction is known as the substrate. The enzyme binds this substrate at the active site, and after converting it into the product, the enzyme remains unchanged and can bind another substrate.
Step-by-step explanation:
The term for a reactant that combines with an enzyme in a reaction is substrate. An enzyme binds to its substrate at a location called the active site. The interaction between an enzyme and its substrate is highly specific; this means the shape and the chemical environment of the active site are uniquely suited for the substrate, similar to a 'lock and key' model. Once the enzyme/substrate complex is formed, the reaction takes place, and the substrate is converted into the product or products. The enzyme itself is not changed by the reaction and can thus continue to catalyze other reactions with a new substrate.