noun
convict; prisoner serving a sentence
Refers specifically to a person currently serving a prison sentence, not just any detainee.
その服役囚は模範的な態度で刑期を短縮された。
The convict had his sentence reduced for exemplary behavior.
Compound of 服役 (penal servitude) + 囚 (prisoner). The exact historical derivation is uncertain, but the spelling is conventionally associated with the meaning.