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. Joules can also be used to measure heat, work, and electricity, making it a versatile unit in physics and engineering. In practical terms, one joule is roughly equivalent to the energy needed to lift a small apple (about 100 grams) one meter off the ground. This unit is named after the English physicist James Prescott Joule, who contributed significantly to the study of energy and thermodynamics.