Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
dimness is a noun meaning "the state of being poorly illuminated". It carries 3 recorded senses.
the state of being poorly illuminated
the property of lights or sounds that lack brilliance or are reduced in intensity
the quality of being dim or lacking contrast
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 dimness come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.