When a light switch is switched off it is an open circuit, the light bulb doesn't have its power flowing so it doesn't light up.

The test is - does the bulb light up.

Andrew

It just means that there is a break in the circuit so that no current can flow through the load. A switch may be open or a fault could cause a conductor to break. Or (in the case of a ring main) the ring may be incomplete so that current flows through only one arm of the ring.

But watch out! An open circuit is not necessarily dead and you could  get a shock. An open circuit is not the same as an isolated circuit, which has all conductors disconnected from the power source.