Receptor
1. specific binding site at the cell membranes and at cellular membranes for substances which serve to transmit signals into the cell interior or into subcellular compartments and to control metabolism (signal transduction).
The basis of R. is the specific receptor protein which, when a corresponding ligand is bound, either opens an ion channel and triggers cell functions, or intervenes in cell metabolic regulation by forming a messenger substance (second messenger) via intermediate reactions.
https://en.wikipedia.org/wiki/Receptor_(biochemistry)
2. sensory nerve cell or elementary sensory organ (e.g.: rod cell in the retina of the eye, or stretch receptor in the aorta and in the carotid sinus (baroreceptor)).