A biological target is an enzyme, receptor or other protein that can be modified by an external stimulus. The definition is context-dependent and can refer to the biological target of a pharmacologically active drug compound, or the receptor target of a hormone (like insulin). The implication is that a molecule is "hit" by a signal and its behavior is thereby changed.

This term is commonly used in pharmaceutical research to describe the native protein in the body that is modified by a medicinal chemical.