The mutants are a race of superpowered individuals that exist in many Universes throughout the Multiverse.
On Earth-515, the term "mutant" is the colloquial term used to describe the Homo superior, a sub-species of humans that are descendants of a group of Homo erectus that were genetically altered by the Celestials over 1,000,000 years ago and were planted with the so-called X-Gene, a special gene which gives these humans a variety of superhuman powers.
A mutant's powers usually manifest in puberty, but sometimes they can manifest earlier or even later than that.
Although the term mutant usually refers to the Homo superior, there are some alien individuals who are considered to be mutant. The Technarch alien Warlock is considered to be a mutant among his kind due to his ability to feel empathy and love.On Earth-38, mutants are every human born with the X-Gene. However, unlike Earth-515, in this Universe the majority of mutants don't naturally awaken their powers with puberty, but rather through an event that triggers their mutant gene - like a Particle Accelerator explosion or a vaccine created specifically to awaken the X-Gene.