c. 1300, "completely, quite, entirely, wholly," c. 1300, from clear (adj.) or adverbial use of the adjective in Old French. From early 14c. as "plainly, lucidly;" mid-14c. as "loudly, with distinctness of sound;" late 14c. as "brightly, brilliantly."
late 14c., "not identical, not the same," also "clearly perceptible by sense," past-participle adjective from obsolete distincten (c. 1300) "to distinguish one thing from another; make distinct," from Old French distincter, from Latin distinctus, past participle of distinguere "to separate between, keep separate, mark off" (see distinguish). Meaning "plain and intelligible to the mind" is from c. 1600. Related: Distinctness.
late 14c., diffinicioun, definicion, "decision, setting of boundaries, determination and stating of the limits and distinctive nature of a thing," also "limitations," also "a statement of the meaning of a word or phrase," from Old French definicion, from Latin definitionem (nominative definitio) "a bounding, a boundary; a limiting, prescribing; a definition, explanation," the last sense most often in Cicero, noun of action from past-participle stem of definire "to limit, determine, explain," from de "completely" (see de-) + finire "to bound, limit," from finis "boundary, end" (see finish (v.)). In logic, meaning "act of stating what something means" is from 1640s. Meaning "degree of distinctness of the details in a picture" is from 1889.