Any cell receptor molecule that binds an opioid (i.e. as its ligand), such as morphine or an endogenous opioid (endorphin, enkephalin, or dynorphin). There are four types, labelled μ‎ (mu), κ‎ (kappa), δ‎ (delta), and nociceptin (the latter has structural similarities to the other three ‘classical’ opioid receptors). All are G-protein-coupled receptors, and their activation through binding of an appropriate ligand leads to analgesia (pain relief), hence their importance in medicine.