Metahuman was a term used to describe humans who are mutated and given extraordinary powers.
Unlike mutants, those who are called metahumans are not born with powers, such as the Avengers and the Fantastic Four. S.H.I.E.L.D. classed the powers of metahumans along with mutants from Gamma to Omega classes. For example, Superman was categorized as a Omega-Class, and Wonder Woman and Flash are considered Alpha-Class metahumans.
Regardless, the term metahuman is universally apply to every superhuman.
In the DC Universe, "metahuman" is designate to humans who gain superpowers through the metagene, a latent gene that is "activated" by stress; once activated, it can carry a parent's powers down to his or her child. In the DC Animated Universe, "metahuman" and "superhero" are used interchangeably. Metahuman is also sometime used as a DC-equivalent of "mutant".
In the animated show Static Shock, Static referred to himself as a metahuman and considered terms, such as "mutant," too derogatory.