Gibbs free energy: the amount of 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.
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.
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