Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
impressiveness is a noun meaning "splendid or imposing in size or appearance". It carries 2 recorded senses.
splendid or imposing in size or appearance
“the grandness of the architecture”
“impressed by the richness of the flora”
the quality of making a strong or vivid impression on the mind
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 impressiveness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.