Opiate: A medication or an illegal drug that is derived from the opium poppy. Opiates are narcotic sedatives that depress activity of the central nervous system, reduce pain, and induce sleep. Codeine, morphine, and heroin are examples of opiates. In contrast, the term opioid is a broader term that originally was used to refer to any substance, natural (such as opiates) or synthetic, that bind to the opioid receptors in the brain and cause sedative and pain relief effects. Some examples of synthetic opioids include the prescription painkillers hydrocodone (Vicodin) and oxycodone (OxyContin), as well as fentanyl and methadone. In recent years, the term opioid has been used more broadly and is often used to refer to the entire class of drugs that consists of both opium derivatives (opiates) and synthetic opioids.
J. Larry Jameson, et al. Harrison's Principles of Medicine. McGraw-Hill Professional Publishing Published: July 23rd 2018; Edition: 20.