Origin and history of unit

late 14c., "numeral below 10," from Latin digitus "finger or toe" (also with secondary meanings relating to counting and numerals), considered to be related to dicere "to say, speak" (from PIE root *deik- "to show," also "pronounce solemnly"). The numerical sense is because numerals under 10 were counted on fingers. The "finger or toe" sense in English is attested from 1640s.

c. 1300, unite, "state or property of being one," from Anglo-French unite, Old French unite "uniqueness, oneness" (c. 1200), from Latin unitatem (nominative unitas) "oneness, sameness, agreement," from unus "one" (from PIE root *oi-no- "one, unique").

Originally in theology, in reference to the persons of the Trinity; general use is by late 14c., "indivisibility, harmonious combination of parts." It is attested by 1580s as "a separate, single thing." In the arts, the unities, in reference the three principles (time, place, action) to be properly maintained in literary works and drama, is by 1660s.

1847, "characterized by unity or uniformity;" 1865, "of or relating to a unit;" see unit + -ary.