Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
uncommonness is a noun meaning "extraordinariness as a consequence of being marked by an uncommon or superlative quality". It carries 2 recorded senses.
extraordinariness as a consequence of being marked by an uncommon or superlative quality
extraordinariness as a consequence of being rare and seldom encountered
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 uncommonness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.