Which term describes a substance that suppresses activity?

The term that describes a substance that suppresses activity is "inhibitor." In the context of chemistry and biology, an inhibitor can be a molecule or compound that slows down or prevents a chemical reaction or a biological process, such as enzyme activity. For example, in enzyme-catalyzed reactions, inhibitors can bind to the enzyme and impede its ability to convert substrates into products, effectively decreasing the rate of the reaction.

Looking at the other options, a catalyst is a substance that accelerates a chemical reaction without being consumed in the process, while a reactant is a substance that undergoes a change during a reaction. An activator, on the other hand, typically refers to a substance that increases the rate of a reaction or enhances the activity of an enzyme or another component. Thus, the role of an inhibitor is distinct in that it specifically functions to suppress or decrease activity.