noun
A somewhat formal or literary term for a prison or jail. In everyday speech, 刑務所 (けいむしょ) or 拘置所 (こうちしょ) are more common for specific institutions.
彼は牢獄から脱走した。
He escaped from prison.
その小説は牢獄での生活を描いている。
The novel depicts life in prison.
Compound of 牢 (ろう, 'prison') and 獄 (ごく, 'jail'). Both characters independently mean a place of confinement, and the combination reinforces the meaning.