What is energy?

Energy is the capacity to do work or produce heat. It exists in many forms – kinetic (motion), potential (position), thermal, chemical, electromagnetic, mass-energy etc. – and can be converted from one to another. A key principle is conservation: total energy in an isolated system is constant. For example, a compressed spring has stored (potential) energy that can do work when released. In relativity, mass itself is a form of energy (E=mc²). Despite its ubiquity, energy is an abstract scalar: one quantifies it via physical processes (lifting weights, heating substances) and tracks it as a bookkeeping of doing work or transferring heat.