n.
Function: noun
Date: 1967
1 : any of a group of endogenous neural polypeptides (as an endorphin or enkephalin) that bind especially to opiate receptors and mimic some of the pharmacological properties of opiates ― called also opioid peptide
2 : a synthetic drug possessing narcotic properties similar to opiates but not derived from opium broadly : OPIATE 1A