Affinity label is a term used to describe a molecule or compound that binds specifically to a target protein or receptor, typically with high affinity. Several synonyms can be used to refer to affinity labels, including ligand, chemical probe, biological assay, affinity reagent, and affinity matrix. These terms are often used interchangeably, but they may carry slightly different connotations depending on the context. For example, a ligand may refer specifically to a small molecule that binds to a receptor, while an affinity matrix may be used to describe a solid support used in chromatography to purify proteins. Regardless of the terminology used, affinity labels are essential tools for understanding the function and regulation of proteins in biology and medicine.