A cavity or surface region on a protein shaped to grip a specific molecule, the physical basis of biochemical recognition. Enzymes bind their substrates at active-site binding pockets, receptors bind hormones and neurotransmitters at theirs, and antibodies bind antigens at theirs, each interaction depending on complementary shape and chemistry. The specificity is exquisite: a binding site typically accepts its target while rejecting thousands of similar molecules. Drugs work by occupying binding sites, either mimicking the natural ligand to activate or blocking it to inhibit. In food science, binding sites explain how proteins carry flavors, how enzymes recognize substrates, and how allergens are recognized by antibodies. Understanding them is understanding molecular communication itself.