WordReference Random House Unabridged Dictionary of American English © 2017
o•pi•oid  pē oid′),USA pronunciation n.  Biochem, Pharm.
  1. Physiology, Biochemistry, Drugsany opiumlike substance.
  2. Physiology, Biochemistry, Drugsany of a group of natural substances, as the endorphins, produced by the body in increased amounts in response to stress and pain.
  3. Biochemistry, Drugsany of several synthetic compounds, as methadone, having effects similar to natural opium alkaloids and their derivatives.

  1. Physiology, Biochemistry, Drugspertaining to such a substance.
  • opi(um) + -oid 1955–60

Collins Concise English Dictionary © HarperCollins Publishers::

opioid /ˈəʊpɪˌɔɪd/ n
  1. any of a group of substances that resemble morphine in their physiological or pharmacological effects, esp in their pain-relieving properties

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