dead, with a hint of sarcasm.
In the most simplest definition, energy is the capacity of doing work, that's what I learnt in school :)
LeetalDeval has it right. Energy is a concept plain and simple that is defined by mass and Einstein's space-time. If you want some mystical metaphysical mumbo jumbo explanation for only $39.95 I sell the secrets to life, the universe, and everything.
Energy is subject to the law of conservation of energy. According to this law, energy can neither be created (produced) nor destroyed by itself. It can only be transformed. Or what you have before you get pregnant