Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
detainment is a noun meaning "a state of being confined (usually for a short time)". It carries one recorded sense.
a state of being confined (usually for a short time)
“his detention was politically motivated”
“the prisoner is on hold”
“he is in the custody of police”
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 detainment come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.