Metahuman (meaning "beyond human") is a broad term describing those humans who have or acquire powers or abilities beyond the normal limits of humanity.
There have been many origins for humans to have superpowers in the various Superman-related comics over the decades, ranging from chemical or radiation exposure, exotic or alien technology, and magic. An individual can be empowered by accident, deliberate action or simply by birth.
In the Post-Crisis DC Universe, the concept of the "meta-gene" was created for the Invasion! series in 1988-89, plotted by Keith Geffen and scripted by Bill Mantlo. It attempted to tie together all the disparate superhero/supervillain origins by positing a single cause for most abilities; a rare genetic code that responded to extreme trauma or life-threatening stress by activating superhuman powers, often incorporating the source of the stress, i.e. a person caught in a fire might develop flame powers. The term "Metahuman" came to replace "superhuman" as a descriptive term for these beings, and over the next few years it expanded in common usage to refer to even those super-powered beings whose abilities explicitly did not come from the meta-gene, such as aliens, magical beings, cyborgs and gods. Occasionally all costumed adventurers are loosely referred to as "metahumans", even those with no powers, but only a high degree of skill and talent.