opioid. 1. any synthetic narcotic that has opiate-like activities but is not derived from opium. 2. denoting naturally occurring peptides, such as enkephalins, that exert opiate-like effects by interacting with opiate receptors of cell membranes.See also opioid analgesic.. any synthetic narcotic that has opiate-like activities but is not derived from opium. 2. denoting naturally occurring peptides, e.g. enkephalins, that exert opiate-like effects by interacting with opiate receptors of cell membranes.