late 14c., "spiritual enlightenment," from Latin illuminationem (nominative illuminatio), from past participle stem of illuminare "to throw into light, make bright, light up;" figuratively "to set off, illustrate," from assimilated form of in- "in, into" (see in-(2)) + lumen (genitive luminis) "light," related to lucere "to shine" (see light, n.). Meaning "action of lighting" is from 1560s.