In scientific terms, the ability or capacity of doing work. Various forms of energy include kinetic, potential, thermal, nuclear, rotational, and electromagnetic. One form of energy may be changed to another, as when coal is burned to produce steam to drive a turbine that produces electric energy.

The ability to do work. 

  1. Potential energy : energy due to the liquid’s location or condition.
  2. Kinetic energy : energy of motion.

Source: Handbook of water and wastewater treatment plant operations