Illustrate from Latin illustrāre (to light up), from lustrāre (to purify by torchlight), PIE *lewk- (light). Three-stage semantic journey: to make famous (1520s) → to clarify with examples (1610s) → to provide pictures (1830s). Third in the *lewk- cluster: lucifer, elucidate, illustrate.
To make clear by examples, figures, or comparisons; to provide a text with pictures or diagrams.
From Latin illustrāre (to light up, to make bright, to make famous or renowned), composed of the intensive prefix in- (here used as an intensifier: thoroughly, completely) and lustrāre (to purify ceremonially by light, to illuminate, to survey), from lustrum (a ceremonial purification by torchlight, held every five years in Rome), from PIE *lewk- (light, brightness, to shine). The PIE root *lewk- produced an extraordinary range of light-words: Latin lūx (light), lūna (moon), lūcifer (light-bearer, Venus as morning star), lūcidus (clear, bright), and via Greek leukós (white, bright), giving English leucocyte. The lustrum ceremony — where sacrificial animals were led around the assembled
When illustrate entered English in the 1520s, it had nothing to do with pictures — it meant 'to make famous,' as if bathing someone in light. 'Clarifying with examples' appeared a century later. 'Providing pictures' didn't emerge until the 1830s. Three meanings in three centuries, all built on the same metaphor: to illuminate the dark.