Various meanings of the term free energy include:

• In science, the term thermodynamic free energy denotes the total amount of energy in a physical system which can be converted to do work.
  • Helmholtz free energy: the amount of thermodynamic energy which can be converted into work at constant temperature and volume. In chemistry, this quantity is called work content.
  • Gibbs free energy: the amount of thermodynamic energy in a fluid system which can be converted into work at constant temperature and pressure. This is the most relevant state function for chemical reactions in open containers.
• The term "free energy" more practically and within engineering fields, usually related to power generation, denotes "free energy" as an energy source available directly from the greater environment and which cannot be expected to be depletable by humans.
  • Free energy may be categorised as renewable energy, although most renewable energy sources would not normally be called free energy sources or sources of perpetual motion.
  • Free energy is energy which may be directly utilized (and returned) by a device from the surroundings (electromagnetic free energy is sometimes referred to as radiant energy).
  • Free energy can also mean a primary energy source that is free (i.e. does not cost anything) for consumption. Examples include wind power, water power, telluric power, and solar power.
• The term "free energy" more generally denotes,
  • Free energy suppression is the notion that corporate energy interests deliberately suppress technologies that may provide energy at very little cost. Remaining so-far-unexploited forces of nature which are well documented in the scientific literature include telluric currents, atmospheric electricity, earth batteries, and pressure system changes.
  • The energy from fantastical forces attributed by fewer "qualified" scientists and engineers, and are skeptically considered perpetual motion. These devices utilize quantum vacuum perturbation (which cannot really be considered to be a force as much as it is a bending of time-space), quantum vacuum energy, rotating magnets (employed in ways hitherto unpracticed), as well as some purported methods to crack hydrogen.
• In biology, free energy pertaining to DNA studies is covered within free energy (DNA)

Freie Energie