This article is from the X-Men Comic Books FAQ, by Kate the Short (firstname.lastname@example.org) with numerous contributions by others.
The main focus of the X-titles is a specific type of character called a
mutant. Forget most of your basic biology when hearing the term "mutant"
applied to a Marvel comic, because the writers usually do. For Marvel
purposes, a mutant is a being who possesses a genetic structure not
present in his parents. While it's useless as a scientific definition
(otherwise, any "non-mutant" child would exactly resemble her parents,
like clones), it's mainly used as a tag for a specific group of
Really, the definition is a bit looser than that, since accurate biology
is usually not the top priority for the writers. For instance, Siryn, is
called a mutant, despite the fact that her powers are the same as those
of her father, Banshee. Some say that Siryn *is* a mutant, in that she
can talk and scream at the same time (it makes perfect sense if you know
the characters), but the main difficulty is bad writing, not bad genes.
The easier way to categorize mutants is to see whether have an active
"X-Factor", and that's really the main point of the whole definition.
You will see references stating that "a mutant has to have a different
power than his parents" in mutant comics, though, so it's mentioned here
just to get you acquainted with it.
So, what are mutants, exactly? They are superhuman because they were
born that way. They didn't need any gamma bomb blowing up, or spider
biting them, or magical formula recited. They're superhuman because
that's what they were born to be. They are mutants because of their X-
Factor. And what is an X-Factor? Read below, true believer!
The reason there are mutants on Earth comes from Marvel cosmology.
Large, alien gods, called Celestials (who some say are but the
incarnations of the dreams of Eternity), visit all planets that will
bear life, early in each planet's existance. They perform genetic
tinkering with the early lifeforms that will, if everything works out
right, leave the species with three distinct superhuman bloodlines:
Eternals (who never suffer random mutations), Deviants (who always
suffer mutations in each generation), and normal folks. In the "normal"
lifeforms, the Celestials left a genetic trigger. Some normals would
gain powers after exposure to odd "triggering" events (like the
Fantastic Four, the Hulk, or Spider-Man). Others could self-trigger when
exposed in the womb to sufficient background radiation. When it's self-
triggered, that genetic trigger is called the X-Factor.
Now the X-Factor only makes a mutant when it's self-triggered. Something
happens to it when it does so that it becomes different than the same
gene that allowed the Fantastic Four to gain their powers; mutants show
up on mutant detectors (which look for the unique signature of the X-
Factor), while Spider-Man doesn't. Mutants also give off unique brain
patterns due to the X-Factor that enable telepaths who know what to look
for (like Professor X) to detect mutants far more easily than normal
humans or non-mutant superheroes. Devices that nullify mutant powers by
negating the X-Factor are useless against non-mutants as well. On the
other hand, Ship (an old base of Apocalypse) had a force field around it
that would only open if it detected the X-Factor inside a visitor. So,
yes, mutants are different than the "normal" superhumans in a Marvel
comic. Aside from that, there's no real appreciable difference or
superiority for mutant superpowers over non-mutant ones. Prejudices,
however, still count most mutants as menaces and most non-mutant
superheroes as friendly (J. Jonah Jameson's views on Spider-Man
So, to sum up: A mutant in the X-Universe is anyone whose powers derive
from the mutant genetic X-Factor introduced into the human race by the
First Celestial Host during prehistory. Got it? Good!