A substance that inhibits cholinesterase, the enzyme that clears the neurotransmitter acetylcholine from nerve junctions, thereby prolonging and intensifying nerve signals. In medicine, these drugs treat conditions such as myasthenia gravis and glaucoma, and they are the basis of Alzheimer’s drugs that modestly preserve memory. In food safety and toxicology, the term is notorious because organophosphate and carbamate insecticides are anticholinesterases: their poisoning of farmworkers and contamination of food produces the classic overstimulation syndrome of sweating, salivation, constricted pupils, and muscle twitching. Residue limits for these pesticides exist precisely to keep dietary exposure far below the doses that inhibit human cholinesterase. The same biochemistry thus spans therapy and toxicology, separated only by dose and intent.