Opiate is a term classically used in pharmacology to mean a drug derived from opium. Opioid, a more modern term, is used to designate all substances, both natural and synthetic, that bind to opioid receptors in the brain (including antagonists).
What are opiates? In medical classification, the group of drugs called “opiates” includes any narcotic opioid alkaloid found as a natural product of the opium poppy plant (lat. papaver somniferum).