Simple Definition:

A watt is a unit of measurement [a standard way to measure something] used to quantify [measure the amount of] the rate at which electrical power is generated or consumed. It is symbolized by the letter “W”. For example, a light bulb that uses 60 watts consumes electrical power at a higher rate than a light bulb that uses 40 watts.

Very Simple Definition:

A watt is a way to measure how fast something uses or makes electricity. For example, a bright light uses more watts than a dim light.