ACETYLCHOLINESTERASE INHIBITOR - A group of medications which exert their effect by inhibiting the activity of acetylcholinesterase, an enzyme which catalyzes the breakdown of the neurotransmitter acetylcholine. This group of medications includes tacrine hydrochloride and donepezil hydrochloride, both of which are used in the treatment of Alzheimer's disease. Another acetylcholinesterase inhibitor is pyridostigmine, used in the treatment of myasthenia gravis.