What is the meaning of Opioid?

A substance that has effects similar to opium:

1. Any of the natural substances, such as an endorphin, released in the body in response to pain; an endogenous opioid.
2. Any of a group of synthetic compounds that exhibit similarities to the opium alkaloids that occur in nature.

   Any of the natural substances, such as an endorphin, released in the body in response to pain; an endogenous opioid.

   Any of a group of synthetic compounds that exhibit similarities to the opium alkaloids that occur in nature.

   Source: wiktionary.org