Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
harmoniousness is a noun meaning "the property of sounding harmonious". It carries 2 recorded senses.
the property of sounding harmonious
compatibility in opinion and action
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 harmoniousness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.