1. An available location on the surface of a catalyst available for reactants to bind and result in a chemical reaction taking place. The active site can be blocked by a chemical agent or poison thereby reducing the effectiveness of the catalyst.
2. On the surface of an enzyme, the active site is the location to which a substrate binds. The binding of the substrate to the enzyme is dependent on the conformation or 3-D shape of the protein. Inhibition prevents the binding from taking place by altering the conformation thereby preventing the substrate from binding, or by blocking the site.