Various meanings of the term free energy include:
- In thermodynamics the term free energy denotes the total amount of energy in a system which can be converted into work.
- In engineering, free energy means an energy source available directly from the environment and which cannot be depleted (so it is available in effectively unlimited quantity). Free energy here is equated with renewable energy, which does not include energy sources which are dependent upon limited resources.
- Free energy, in a strict sense, is energy directly transceived from the environment, and utilized without any artificial aid (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. The term "free energy" in this sense is usually associated with hypothetical perpetual motion machines, and used by proponents of these machines.
- In conspiracy theories, free energy suppression is the notion that corporate energy interests deliberately suppress technology that would either provide energy at very little cost (like the above mentioned perpetual motion machines), or reduce energy use to negligible levels. Examples include the allegations that light-bulb companies secretly own the patents to light bulb designs that never burn out, or that fuel companies are sitting on designs for highly efficient vehicles.
- For free energy pertaining to DNA studies, see Free energy (DNA)