Dictionary.net

trade names of a narcotic analgesic that can be inhaled and that acts on the central nervous system and may become addictive; used as a veterinary anesthetic and with other drugs before, during, and after surgery.

A potent narcotic analgesic, abuse of which leads to habituation or addiction. It is primarily a mu-opioid agonist. Fentanyl is also used as an adjunct to general anesthetics, and as an anesthetic for induction and maintenance. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1078)