Lights something up or makes it brighter; can also mean to make something clearer or easier to understand.
From Latin 'illuminare' meaning 'to light up,' composed of 'in-' (in) and 'luminare' (to brighten, from 'lumen' meaning light). It entered English in the 13th century with both literal and metaphorical meanings.
The same root 'lumen' that appears in 'illuminate' is used today to measure brightness—a lumen is still the scientific unit for how much light actually reaches your eyes!
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