illustrateyoudaoicibaDictYouDict[illustrate 词源字典]
illustrate: [16] Illustrate is closely related etymologically to illuminate. It goes back to Latin illustrāre, a compound verb formed from the prefix in- and lustrāre ‘make bright’, which came from the same base as produced Latin lūmen (source of illuminate) and lūx ‘light’, and indeed English light. Originally it meant literally ‘throw light on’, but this eventually passed via ‘elucidate’ to, in the 17th century, ‘exemplify’ and ‘add pictures to’.

More of the original sense of ‘brightness’ survives, albeit metaphorically, in illustrious [16], which comes from Latin illustris ‘shining, clear’, a back-formation from illustrāre.
=> illuminate, illustrious, light, luminous, lustre[illustrate etymology, illustrate origin, 英语词源]
illustrate (v.)youdaoicibaDictYouDict
1520s, "light up, shed light on;" 1610s, "educate by means of examples," back-formation from illustration, and in some cases from Latin illustratus, past participle of illustrare (see illustration). Sense of "provide pictures to explain or decorate" is 1630s. Related: Illustrated; illustrating.