noun A synthetic opioid narcotic analgesic, C22H28N2O, with pharmacological action similar to morphine that is administered transdermally as a skin patch and in the form of its citrate, C22H28N2O·C6H8O7, is administered orally or parenterally (as by intravenous or epidural injection); N-phenyl-N-[1-(2-phenylethyl)piperidin-4-yl]propanamide.