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1560s, "single number regarded as an undivided whole," alteration of unity on the basis of digit. Popularized in John Dee's English translation of Euclid, to express Greek monas (Dee says unity formerly was used in this sense). Meaning "single thing regarded as a member of a group" is attested from 1640s. Extended sense of "a quantity adopted as a standard of measure" is from 1738. Sense of "group of wards in a hospital" is attested from 1893.
unit u·nit (yōō'nĭt)
An entity regarded as an elementary structural or functional constituent of a whole.
A precisely specified quantity in terms of which the magnitudes of other quantities of the same kind can be stated.
The quantity of a serum, drug, or other agent necessary to produce a specific effect.