An enzyme whose synthesis (rate of production) is inhibited (repressed) when the product that it (or it in a multienzyme sequence) synthesizes is present in high concentrations. It is a way of shutting down the synthesis of an enzyme whose product is not required because so much of it is readily available to the cell. When that enzyme product is no longer available (e.g., because the cell has consumed that product) more of the enzyme is synthesized (to catalyze production of more product).