A joule is a unit of energy in the International System of Units (SI). It is defined as the amount of energy transferred when a force of one newton moves an object one meter in the direction of the force. The joule can also be expressed in terms of other units, such as calories or watt-hours, making it versatile for various scientific applications. In practical terms, one joule is roughly equivalent to the energy needed to lift a small apple (about 100 grams) one meter against the force of gravity. This unit is named after the English physicist James Prescott Joule, who contributed significantly to the study of energy and thermodynamics.