noun Biochemistry, Pharmacology.
any opiumlike substance.
any of a group of natural substances, as the endorphins, produced by the body in increased amounts in response to stress and pain.
any of several synthetic compounds, as methadone, having effects similar to natural opium alkaloids and their derivatives.
pertaining to such a substance.

1955–60; opi(um) + -oid

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World English Dictionary
opioid (ˈəʊpɪˌɔɪd)
a.  any of a group of substances that resemble morphine in their physiological or pharmacological effects, esp in their pain-relieving properties
 b.  (modifier) of or relating to such substances: opioid receptor; opioid analgesic

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Word Origin & History

1957, from opium + -oid.
Online Etymology Dictionary, © 2010 Douglas Harper
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American Heritage
Medical Dictionary

opioid o·pi·oid (ō'pē-oid')
See opiate. adj.

o'pi·oid' adj.
The American Heritage® Stedman's Medical Dictionary
Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.
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Example sentences
That's good, because fat activates the brain's natural mu-opioid receptors, provoking what scientists call a hedonic response.
Natural pain moderation depends on the activation of a brain cell surface protein called the mu-opioid receptor.
Morphine acts on a part of the brain known as the opioid system, which is
  linked to pain, pleasure and addictive behaviors.
It is no more neurotoxic than any other opiate, or the opioid substances
  produced in the brain naturally.
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