What does energy mean?
Energy refers most broadly to the strength and vitality that allows a person or system to sustain physical or mental activity — the vigor behind a brisk morning run or a demanding day of work. In science, it takes on a precise meaning: the property of matter and radiation that manifests as a capacity to perform work, whether kinetic, thermal, electrical, chemical, or nuclear. A cornerstone of modern physics is the law of conservation of energy, which holds that energy cannot be created or destroyed, only transformed from one form into another. In everyday speech, the word also carries connotations of enthusiasm and dynamism, describing people who approach life with drive and intensity. Derived from the Greek 'energeia', meaning activity or operation, the term entered English in the sixteenth century and has since become one of the most versatile and essential words in both scientific and ordinary vocabulary.
nounThe strength and vitality required for sustained physical or mental activity; in physics, the property of matter and radiation that is manifest as a capacity to perform work (such as causing motion or the interaction of molecules).
- Strength, vigor, or sustained physical and mental capacity for activity.
- Power derived from physical or chemical resources, especially as harnessed to provide light, heat, or drive machines.
- (Physics) The measurable property of a system that enables it to do work; conserved in all processes per the law of conservation of energy.
- A person's characteristic enthusiasm, forcefulness, or intensity of expression.
"She arrived at the meeting brimming with energy despite the early start."
"The children burned off their excess energy in the park."
"Solar panels convert sunlight into usable electrical energy."
Usually uncountable in everyday use; the plural typically refers to distinct kinds of effort or vitality directed at particular activities.
"She channeled her creative energies into painting after retiring."
Aristotle used 'energeia' to mean being-at-work — and 2,000 years later, physics still can't tell you exactly what energy *is*, only how it behaves.
Reviewed by Deb Chak, Editor. AI-assisted content curated by RJS Tech Solutions LLP.
Etymology of energy
Energy derives ultimately from the Greek 'energeia', meaning activity or operation, formed from 'ergon' (work) — the same root that gives us 'erg', a unit of energy. Aristotle employed 'energeia' philosophically to denote actuality, the state of being at work rather than merely potential. The word passed into Latin as 'energia' before entering English in the mid-sixteenth century, initially in reference to vigorous expression or forceful style. Its scientific sense developed later, becoming central to physics during the nineteenth century as thermodynamics matured.
Related word forms
How energy is actually used
In everyday use, 'energy' often appears with quantifiers like 'a lot of' or 'plenty of' and is typically uncountable in this sense. In physics, it is a precisely defined technical term. The plural 'energies' usually refers to different kinds of effort or vitality directed at particular pursuits.
Easily confused with energy
Despite looking similar, 'enervate' means to drain of vitality — nearly the opposite of energizing.