opioidPronunciation: /ˈəʊpɪɔɪd/ BiochemistryDefinition of opioid noun a compound resembling opium in addictive properties or physiological effects: opioids other than heroin adjective relating to or denoting opioids: opioid useOrigin: 1950s: from opium + -oidopioid in other Oxford dictionaries Definition of opioid in the US English dictionary