Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
illustriousness is a noun meaning "the property possessed by something or someone of outstanding importance or eminence". It carries one recorded sense.
the property possessed by something or someone of outstanding importance or eminence
Forms marked · are formed by the regular rule rather than recorded in the source data. English keeps a few hundred irregulars and the rest follow the pattern, so the rule is right far more often than not — but it is a rule, and the mark says so.
Definitions, synonyms and semantic relations for illustriousness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.