noun
General term for a jail or prison; can refer to the building or the institution. Slightly colloquial compared to 刑務所.
彼は牢屋に入れられた。
He was put in jail.
牢屋から脱走した囚人がまだ捕まっていない。
The prisoner who escaped from jail has not been caught yet.
Compound of 牢 (ろう, 'prison') and 屋 (や, 'house/shop'). The exact historical derivation is uncertain, but the combination straightforwardly means 'prison building'.