1. a natural, semisynthetic, or synthetic substance that typically binds to the same cell receptors as opium and produces similar narcotic effects (such as sedation, pain relief, slowed breathing, and euphoria):

2. any of various of endogenous polypeptides (such as an endorphin or enkephalin) produced by neurons of the peripheral and central nervous system
The people who rated pain the lowest were those whose brains began producing natural painkillers called opioids the fastest …
— John O'Neil

3. any of various opiates (such as morphine), semisynthetic opiate derivatives (such as heroin, hydrocodone, or oxycodone), or synthetic preparations (such as fentanyl or methadone) that may be used illicitly for their narcotic properties and are associated with physiological tolerance (see tolerance sense 4a(1)), physical and psychological dependence, or addiction upon repeated or prolonged use
… fentanyl, a synthetic opioid at least 75 times more potent than morphine.
— Ryan Trimble and Eric S. Peterson Naltrexone is an opioid antagonist used for the treatment of heroin addiction.
— Scientific American The prototypical opioids are morphine and codeine (which is milder than morphine).
— Harvard Health Letter When Walker County was identified as the epicenter of Alabama's opioid crisis, it was no surprise to residents who watched as drugs brought death and devastation down upon their families, neighbors and communities.
— Ashley Remkus Note: The word opioid was originally used only for morphine-like substances not derived from opium, but it has now become widely accepted as a broader term encompassing any substance—natural or synthetic, opium-derived or not—that binds to opiate cell receptors and induces sedation, analgesia, and euphoria.

4. possessing narcotic properties characteristic of opiates of, relating to, involving, or being an opioid
opioid drugs opioid addiction endogenous opioid endorphins opioid cell receptors

5. possessing narcotic properties characteristic of opiates of, relating to, involving, or being an opioid
opioid drugs opioid addiction endogenous opioid endorphins opioid cell receptors

6. a natural, semisynthetic, or synthetic substance that typically binds to the same cell receptors as opium and produces similar narcotic effects (as sedation, pain relief, slowed breathing, and euphoria):

7. any of various of endogenous polypeptides (as an endorphin or enkephalin) produced by neurons of the peripheral and central nervous system

8. any of various opiates (as morphine), semisynthetic opiate derivatives (as heroin, hydrocodone, or oxycodone), or synthetic preparations (as fentanyl or methadone) that may be used illicitly for their narcotic properties and are associated with physiological tolerance (see tolerance sense 1), physical and psychological dependence, or addiction upon repeated or prolonged use
… fentanyl, a synthetic opioid at least 75 times more potent than morphine.
— Ryan Trimble and Eric S. Peterson The prototypical opioids are morphine and codeine (which is milder than morphine).
— Harvard Health Letter Note: The word opioid was originally used only for morphine-like substances not derived from opium, but it has now become widely accepted as a broader term encompassing any substance—natural or synthetic, opium-derived or not—that binds to opiate cell receptors and induces sedation, analgesia, and euphoria.