An exosite is a secondary binding site, remote from the active site, on an enzyme

Enzyme

Enzymes are proteins that catalyze chemical reactions. In enzymatic reactions, the molecules at the beginning of the process, called substrates, are converted into different molecules, called products. Almost all chemical reactions in a biological cell need enzymes in order to occur at rates...

 or other protein

Protein

Proteins are biochemical compounds consisting of one or more polypeptides typically folded into a globular or fibrous form, facilitating a biological function. A polypeptide is a single linear polymer chain of amino acids bonded together by peptide bonds between the carboxyl and amino groups of...

.

This is similar to allosteric sites, but differs in the fact that, in order for an enzyme to be active, its exosite typically must be occupied. Exosites have recently become a hot topic in biomedical research as potential drug targets.