Metahumans is a catchall term for any extraordinary individual.
One of the most common origins for metahumans is a latent gene that developed into a power that defies what we know to be the laws of reality. Whether this is super-speed, inhuman strength or even the ability to fly these abilities are sometimes utilised by people who don masks and capes to help people, or themselves. Others try their best to live a normal life with whatever boons or hindrances their powers bring them.
Beyond what we currently understand to be science, whether this is genetic modification, nanobots that reinforce the skin to be as strong as metal, shrink rays, power-suits or really big guns, super-science encompasses all metahumans who use these technological marvels to become more than human.
At the edge of human understanding is the supernatural, where magic makes the impossible... well possible.
Often overlapping with super-science, alien origins can make people more than human, unless they are an alien in which case metahuman is only a loose term to describe them.
Of course alien can even overlap with magic when we consider the truly alien extradimensional beings.