"Metahuman" is a term coined by the alien Dominators. Originally envisioned as people with a genetic mutation (being on par with Mutants in the Marvel Universe, and Post-Human and Gen-Active in the Wildstorm Universe), it has since found more wider application and is used to describe any human being with what are commonly described as "super powers".
The prefix meta-, in this context, simply means "beyond"—as in metastable, which is "beyond regular stability; ready to collapse at the slightest disruption", or metamorphosis, which is "the state of going beyond a single shape". So, "metahuman" simply describes persons and/or abilities beyond normal human limits.
The terms "meta" and "metahuman" do not exclusively 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 some instances as "metahumans", and even Batman was once called one. It can apparently be used to refer to anyone with extranormal powers, no matter the origins and including those not born with such power.
The term "mutant" has sometimes been used to describe humans who were born with super-powers that manifested spontaneously, without any apparent catalyst. This term was mostly used before the discovery of the meta-gene. It is unclear if so-called mutants like Captain Comet or Jericho are metahumans or not.
By the definition that Checkmate uses, 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.
One of the Dominators discovered that some humans had a "biological variant" he called the meta-gene. 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, from which her famed ultrasonic scream, known as the Canary Cry, developed.
Some metahumans may be born with incredible and incomprehensible powers and abilities. Examples of powers shared by many individuals include telepathy, telekinesis, flight, the ability to project energy, accelerated healing; and enhanced physical strength, agility, or senses—all to variable limits. Other metahumans may also experience a minor to radical alteration in physical appearance from the human norm—wings, gills, a tail, fur, altered skin-color, etc.
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 onward, the Sons of Vulcan passed the metavirus down in an unbroken line, sworn to hunt and kill White Martians.
There exists a toxic mutagen called the Exo-gene. 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.
Population: 1.3 million