(pharmacology) A sedative of the benzodiazepine class, used (as the hydrochloride) chiefly as a premedication for anesthesia; 8-chloro-6-(2-fluorophenyl)-1-methyl-4H-imidazo[1,5-a][1,4]benzodiazepine, C₁₈H₁₃ClFN₃.