The tap or valve on a pipe that you turn to shut off, or let through, the water or gas inside. It is not the tap you wash your hands at — it is the control one, usually hidden: under the kitchen sink, in a cupboard, out by the street under a small metal cover. Every house has one, and the moment you need it is the moment a pipe bursts, which is why plumbers in Britain ask 'do you know where your stopcock is?' The word is mainly British; American plumbers usually say shut-off valve or the main. It sounds technical but ordinary people use it — it is the word on the plumber's bill and in the letter from the water company. There is also a rare verb, to stopcock something, meaning to fit or close such a valve, but you will almost never meet it.

水閥、總開關（水管或瓦斯管上的止水閥）；台灣一般講「水表旁的總開關」或「止水閥」