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late 14c., "decision, setting of boundaries," from Old French definicion, from Latin definitionem (nominative definitio), noun of action from past participle stem of definire (see define).
In logic, meaning "act of stating what something means" is from 1640s; meaning "a statement of the essential nature of something" is from late 14c.; the special focus on words developed after c.1550. Meaning "degree of distinctness of the details in a picture" is from 1889.