c.1400, "a shining;" early 15c., "a manifestation;" mid-15c., "a spiritual illumination," from Old French illustration "apparition, appearance," and directly from Latin illustrationem (nominative illustratio) "vivid representation" (in writing), literally "an enlightening," from past participle stem of illustrare "light up, make light, illuminate;" figuratively "make clear, disclose, explain; adorn, render distinguished," from assimilated form of in- "in" (see in-(2)) + lustrare "make bright, illuminate," related to lucere "shine," lux "light" (see light, n.). Mental sense of "act of making clear in the mind" is from 1580s. Meaning "an illustrative picture" is from 1816.