From Longman Dictionary of Contemporary Englishcellmatecell‧mate /ˈselmeɪt/ noun [countable] someone who shares a prison cell with someone else
Examples from the Corpus
cellmate• With the departure of her cellmate, Fourth Aunt experienced the anxieties of loneliness.• Her cellmate said nothing as she lay motionless on her cot, staring at the ceiling.• He and his cellmate were repeatedly tortured.• This was Bobby Dupard, his cellmate in the brig in Atsugi.• His cellmate slept through the tragedy.• Martin and Gail are more like cellmates than soulmates.• Soon after being remanded to Pentonville Prison he was attacked by his psychotic cellmate who had set fire to the cell. 