Metahumans are a kind of humans that have developed superpowers.
One of the Dominators discovered that some humans had a "biological variant" he called the meta-gene (also spelled "metagene"). This gene often lay dormant until a moment of extraordinary physiological stress activated it, and upon activation it would use the source of the biostress as a catalyst for "genetic change," resulting in metahuman abilities. There are also people born with superhuman abilities, suggesting the metagene can activate spontaneously and without any prior appearance in the ancestry. One well-known example involves Dinah Laurel Lance, the second Black Canary. Although her mother (Dinah Drake Lance, the original Black Canary) was a superhero, neither she nor her husband Larry Lance were born with any known metagenes. However, Dinah Laurel was born with a metagene; the famed ultrasonic scream known as the Canary Cry. The prefix meta-, in this context, simply means "beyond"—as in metastable, which is beyond regular stability and ready to collapse at the slightest disruption, or metamorphosis, which is the state of going beyond a single shape. Dominators point out that the location of the Meta-gene is somewhere near the brain. (Of course, in reality every cell in the body would contain this gene.) Metahuman criminals are incarcerated in special metahuman prisons, like the one built on Alcatraz Island, which outfitted not only with provisions to hold criminals whose powers are science and technology-based, but even mystical dampeners to hold villains (including Homo Magi) whose powers are magic based. Prisoners in this facility are tagged with nanobyte tracers injected into their bloodstream that allows them to be located wherever they are.
The genetic potential for a future metagene was discovered in ancient human DNA by the White Martian race. The White Martians performed experiments on these primitive humans, changing the metagene. Due to their experimentations, they actually altered the destiny of the human race. Whereas before evolution would have eventually made mankind into a race of superhumans similar to the Daxamites and Kryptonians, now only a select few humans would be able to develop metahuman powers. As punishment for this, the group of renegades known as the Hyperclan was exiled to the Still Zone, a version of the Phantom Zone.
The White Martians also created a metavirus, a metagene that could be passed from host to host via touch. This metavirus was responsible for the empowerment of the very first Son of Vulcan. And from that time onwards the Sons of Vulcan passed the metavirus down in an unbroken line, sworn to hunt and kill White Martians.
The terms "meta" and "metahuman" does not only refer to humans born with biological variants. Superman and Martian Manhunter (aliens) as well as Wonder Woman (a near-goddess) and Aquaman (an Atlantean) are referred to in many instances as "metahumans." It can refer to anyone with extranormal powers, no matter the origins and including those not born with such power. According to Countdown to Infinite Crisis, there are roughly 1.3 million metahumans on Earth, 99.5% of which are considered "nuisance-level" (such as kids who can bend spoons with their mind and the old lady "who keeps hitting at Powerball"). The other 0.5% are what Checkmate and the O.M.A.C.s consider alpha and beta level threats. For example, Superman and Wonder Woman were categorized as alpha level, while Metamorpho was considered a beta.
There exists a toxic mutagen called the Exo-gene (also referred to as the Exogene). It is a toxic gene therapy treatment created by Lexcorp for the Everyman Project which creates metahuman abilities in compatible non-metahumans. . The project was controversial, creating a lot of unstable heroes and gave Luthor an "off switch" for their powers, creating countless mid-flight deaths.
Some humans have inherent ability to utilize magic, and these humans are part of a branch or offshoot of humanity referred to as the "Homo Magi", who have interbred with normal humans. As with aliens and mutants with superhuman powers, homo magi are also often classed together as Metas by the general public.