/oh"pee oyd'/ , n. Biochem, Pharm.
1. any opiumlike substance.
2. any of a group of natural substances, as the endorphins, produced by the body in increased amounts in response to stress and pain.
3. any of several synthetic compounds, as methadone, having effects similar to natural opium alkaloids and their derivatives.
adj.
4. pertaining to such a substance.
[ 1955-60; OPI(UM) + -OID ]