Energy is a fundamental concept in physics and the natural sciences, often described as the capacity to do work. At its simplest, energy is the ability to cause change or perform an action. It is a scalar quantity, meaning it has magnitude but no direction, and it is conserved in isolated systems, meaning it cannot be created or destroyed, only transformed from one form to another.

Energy manifests in various forms, including kinetic energy (the energy of motion), potential energy (stored energy due to position or configuration), thermal energy (heat), electrical energy, chemical energy, nuclear energy, and more. Each form of energy can be converted into another, and this transformation is governed by the laws of thermodynamics.

For example, when you lift a book from the ground to a shelf, you are doing work against gravity, converting chemical energy from your muscles into potential energy stored in the book's elevated position. When you drop the book, that potential energy is converted into kinetic energy as the book falls, and upon impact, it may be transformed into sound energy and thermal energy.

In everyday language, energy is often associated with vitality or the capacity for vigorous activity, such as when we say someone has a lot of energy. However, in the scientific context, energy is a precise and quantifiable concept that underpins much of our understanding of the physical world.

The unit of energy in the International System of Units (SI) is the joule (J), named after the English physicist James Prescott Joule. One joule is defined as the amount of work done when a force of one newton displaces an object by one meter in the direction of the force.

In summary, the simplest definition of energy is the capacity to perform work or cause change, and it is a conserved quantity that can take many forms and be transformed from one to another.