Simple Definition:

A volt meter is a device used to measure the electrical voltage [the amount of electrical force] in a circuit. It consists of a display screen [a monitor that shows information] that indicates the voltage level and two probes [thin metal sticks] that are connected to the circuit to measure the voltage. For example, a volt meter can be used to measure the voltage of a battery or the electrical outlet in a wall.

Very Simple Definition:

A volt meter is a device that measures how strong the electricity is in something. For example, it can tell you how much electricity is in a battery or in the wall socket.