The general meaning of energy is the ability to be active. If you have a lot of energy, it means you like to be active. If you plan a low-energy day, it means a day of lounging around.

Energy is from Greek energeia "activity, operation," from energos "active, effective," from en "in" plus ergon "work." Specialized senses of the word energy include the power that comes from sources such as heat or electricity, and the ability of sources such as heat or light to do work. In physics, an erg is the centimeter-gram-second unit used to measure the amount of energy or the amount of work done.

Definitions of energy
  1. noun
    forceful exertion
    “he plays tennis with great energy
    synonyms: vigor, vigour, zip
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    athleticism, strenuosity
    intense energy
    type of:
    force, forcefulness, strength
    physical energy or intensity
  2. noun
    enterprising or ambitious drive
    “Europeans often laugh at American energy
    synonyms: get-up-and-go, push
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    second wind
    renewed energy or strength to continue an undertaking
    type of:
    the trait of being highly motivated
  3. noun
    a healthy capacity for vigorous activity
    “jogging works off my excess energy
    synonyms: vim, vitality
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    energetic vitality
    ch'i, chi, ki, qi
    the circulating life energy that in Chinese philosophy is thought to be inherent in all things; in traditional Chinese medicine the balance of negative and positive forms in the body is believed to be essential for good health
    type of:
    good health, healthiness
    the state of being vigorous and free from bodily or mental disease
  4. noun
    an imaginative lively style (especially style of writing)
    “his writing conveys great energy
    synonyms: muscularity, vigor, vigour, vim
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    verve, vitality
    an energetic style
    light, spark, sparkle, twinkle
    merriment expressed by a brightness or gleam or animation of countenance
    type of:
    life, liveliness, spirit, sprightliness
    animation and energy in action or expression
  5. noun
    any source of usable power
    “the DOE is responsible for maintaining the energy policy”
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    type of:
    physical phenomenon
    a natural phenomenon involving the physical properties of matter and energy
  6. noun
    (physics) a thermodynamic quantity equivalent to the capacity of a physical system to do work; the units of energy are joules or ergs
    energy can take a wide variety of forms”
    synonyms: free energy
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    show 32 types...
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    activation energy, energy of activation
    the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur
    alternative energy
    energy derived from sources that do not use up natural resources or harm the environment
    atomic energy, nuclear energy
    the energy released by a nuclear reaction
    binding energy, separation energy
    the energy required to separate particles from a molecule or atom or nucleus; equals the mass defect
    chemical energy
    that part of the energy in a substance that can be released by a chemical reaction
    electrical energy, electricity
    energy made available by the flow of electric charge through a conductor
    energy level, energy state
    a definite stable energy that a physical system can have; used especially of the state of electrons in atoms or molecules
    rest energy
    the energy equivalent to the mass of a particle at rest in an inertial frame of reference; equal to the rest mass times the square of the speed of light
    (physics) a manifestation of energy; the transfer of energy from one physical system to another expressed as the product of a force and the distance through which it moves a body in the direction of that force
    heat, heat energy
    a form of energy that is transferred by a difference in temperature
    mechanical energy
    energy in a mechanical form
    radiant energy
    energy that is transmitted in the form of (electromagnetic) radiation; energy that exists in the absence of matter
    energy that is radiated or transmitted in the form of rays or waves or particles
    AC, alternating current, alternating electric current
    an electric current that reverses direction sinusoidally
    atomic power, nuclear power
    nuclear energy regarded as a source of electricity for the power grid (for civilian use)
    ionizing radiation
    high-energy radiation capable of producing ionization in substances through which it passes
    cosmic radiation
    radiation coming from outside the solar system
    DC, direct current, direct electric current
    an electric current that flows in one direction steadily
    an electric quantity (voltage or current or field strength) whose modulation represents coded information about the source from which it comes
    electromagnetic radiation, electromagnetic wave, nonparticulate radiation
    radiation consisting of waves of energy associated with electric and magnetic fields resulting from the acceleration of an electric charge
    geothermal energy
    energy derived from the heat in the interior of the earth
    K.E., kinetic energy
    the mechanical energy that a body has by virtue of its motion
    heat of dissociation
    the heat required for a fluid substance to break up into simpler constituents
    heat of formation
    the heat evolved or absorbed during the formation of one mole of a substance from its component elements
    heat of solution
    the heat evolved or absorbed when one mole of a substance is dissolved in a large volume of a solvent
    heat of transformation, latent heat
    heat absorbed or radiated during a change of phase at a constant temperature and pressure
    specific heat
    the heat required to raise the temperature of one gram of a substance one degree centigrade
    luminous energy
    the energy associated with visible light
    P.E., potential energy
    the mechanical energy that a body has by virtue of its position; stored energy
    solar energy, solar power
    energy from the sun that is converted into thermal or electrical energy
    solar radiation
    radiation from the sun
    wind generation, wind power
    power derived from the wind (as by windmills)
    type of:
    physical phenomenon
    a natural phenomenon involving the physical properties of matter and energy
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