noun · a device such as a power switch or gas valve used to control the flow of electricity or fluid

那个电灯的开关在墙角。

noun · the action of switching something on and off or opening and closing it

频繁开关电脑会缩短其使用寿命。

verb · the act of opening the city or frontier gate

这座古城的城门每天早晚都会定时开关。