noun

1. 1. the smallest whole number; one
   2. a magnitude or number regarded as an undivided whole
   3. the number in the position just to the left of the decimal point
2. any fixed quantity, amount, distance, measure, etc. used as a standard; specif.,
   1. ☆ a fixed amount of work used as a basis in awarding scholastic credits, usually determined by the number of hours spent in class
   2. the amount of a drug, vaccine, serum, or antigen needed to produce a given result, as on a certain animal or on animal tissues
3. 1. a single person, group, or thing, esp. as distinguished from others or as part of a whole
   2. a single, distinct part or object, esp. one used for a specific purpose the lens unit of a camera
   3. a single residence, as an apartment, that is part of a complex a rental unit
4. Mil. an organized body of troops, airplanes, etc. forming a subdivision of a larger body

Etymology: back-form. (prob. modeled on digit) < unity