Simple Definition:

Power is a measure of how fast work is done. It is usually measured in watts. Work is the amount of energy used to move an object. Power is calculated by dividing the amount of work done by the time it takes to do it. For example, lifting a weight requires a certain amount of work, and doing it quickly requires more power. One joule [a unit measuring work] of work done in one second is equal to one watt (1 watt = 1 joule/second).

Very Simple Definition:

Power is how fast something can do a job. It tells us how quickly something can change the way things are. For example, a strong person can lift heavy things faster than a weaker person, so the strong person has more power.