1 a: the first and least natural number :oneb: a single quantity regarded as a whole in calculation 2: a determinate quantity (as of length, time, heat, or value) adopted as a standard of measurement: as a: an amount of work used in education in calculating student credits b: an amount of a biologically active agent (as a drug or antigen) required to produce a specific result — compare international unit 3 a: a single thing, person, or group that is a constituent of a whole b: a part of a military establishment that has a prescribed organization (as of personnel and materiel) c: a piece or complex of apparatus serving to perform one particular function d: a part of a school course focusing on a central theme e: a local congregation of Jehovah's Witnesses f: a small molecule especially when combined in a larger molecule <repeating units of a polymer>g: an area in a medical facility and especially a hospital that is specially staffed and equipped to provide a particular type of care <an intensive care unit>
