This term refers to a situation in a biochemical reaction in which the action of an enzyme is inhibited by the presence of another molecule.
In a competitive inhibition, the interfering molecule attracts the enzyme away from its intended substrate and thus reduces the rate of catalysis by reducing the number of enzyme molecules available for the reaction. The enzyme can either bind to its substrate, or the inhibiting molecule, but not both. This type of inhibition is reversible.
From the BioTech Dictionary at http://biotech.icmb.utexas.edu/. For further information see the BioTech homenode.