Metahumans (also spelt Meta-Humans and sometimes reffered to as Superhumans or Metas) are humans with what are commonly called "super powers". The term "meta" meaning "beyond" describing people and powers as beyond human limits.
All "natural" Metahumans have a biological variant, called the Metagene. It is the Metagene which activates powers within oneself. How in fact the Metagene occurs is still a mystery. Some believe it to be passed genetically, from one generation to another, but there are cases where children of Metahumans do not have any powers, or vice versa.
Powers the Metagene activates include flight, telepathy, telekinesis, enhanced physical strength, accelerated healing, superhuman speed, and in some cases, the Metagene has exposed a minor to radical alteration in physical appearance from the human norm - wings, gills, a tail, fur, altered skin color, etc.
Around the world more and more Metahumans are emerging everyday. 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. 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 considered alpha and beta level threats. For example Superman and Wonder Woman are alpha level, whilst Starfire and Beast Boy are only beta level.
Genetically Altered Metahumans
It is possible for a human to be genetically modified to become a Metahuman. 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.