1. A cell or group of cells specialized to detect a particular stimulus and to initiate the transmission of impulses via the sensory nerves. The eyes, ears, nose, skin, and other sense organs all contain specific receptors responding to external stimuli (see exteroceptor); other receptors are sensitive to changes within the body (see interoceptor). See also baroreceptor; chemoreceptor; electroreceptor; magnetoreceptor; mechanoreceptor; osmoreceptor; proprioceptor; thermoreceptor.

2. A protein that can bind with a specific ligand (i.e. a hormone, neurotransmitter, drug, or other chemical), thereby initiating a change within the cell. Some receptor proteins occur in the plasma membrane, typically spanning the membrane and having outward- and inward-facing sites. These bind large or polar ligands that cannot cross the plasma membrane. Other receptors are intracellular, located in the cytoplasm or inside the nucleus. They bind small or nonpolar ligands that are able to diffuse across the plasma membrane. Many intracellular receptors are transcription factors; when activated they move into the nucleus to regulate the expression of specific genes. Functionally, receptors fall into two main categories: ionotropic receptors and metabotropic receptors.