The noun sense names one of the five night-watch periods of imperial China, each about two hours:
一更 19-21,
二更 21-23,
三更 23-01,
四更 01-03,
五更 03-05. Watchmen struck a wooden
梆 clapper to mark each one — the act called
打更. The system is gone but the phrase
三更半夜 ("dead of night") survives in everyday Chinese.