What is the definition of energy?

Answer 1

Energy is the ability to cause change in matter or the environment.

Energy is a force that can create a reaction or motion.
It is hard to give a definition that is both accurate and easy to understand. It is sometimes defined as "the ability to do work", but this really doesn't cover all cases. There is a certain magnitude, called "energy", that can be calculated for physical systems; this magnitude is known to be conserved, meaning that it doesn't increase or decrease over time. For a more detailed explanation, it is best to read the Wikipedia article on "Energy", or some similar overview.

Answer 2

Energy is the ability to do work or cause change. It exists in many different forms, such as heat, light, sound, and kinetic energy. Energy cannot be created or destroyed, but it can be transferred or converted from one form to another.