Syllable division derived from spelling — this word is not in the pronouncing dictionary, so no phonetic transcription is shown rather than a guessed one.
foster-son is a noun meaning "someone who is raised as a son although not related by birth". It carries one recorded sense.
someone who is raised as a son although not related by birth
Definitions, synonyms and semantic relations for foster-son come from Princeton University’s WordNet. Commonness is measured against a corpus of everyday English. Full sources and licences.