In X-Men, Wolverine, Magneto, Mystique, Beast, etc. are all mutants. Mutants are perhaps the people in the form of humans who have extraordinary powers. So, in that terms, why aren't Iron Man, Captain America, Ant-Man, or in DC Comics, Superman, Batman, Flash considered to be mutants? Is the term mutant reserved only for X-Men?
Iron Man (in the MCU, at least) is just a genetically normal person with lots of money and technology. Sure, he had an arc reactor in his chest for a while, but he's no mutant. A mutant as seen in the X-Men would be born genetically different from a normal human.
As noted in the above answer, Captain America was a normal person before being subjected to the process which turned him into a superhero. It is often the case with Marvel heroes such as the Hulk or Spider-Man that they were normal before being given superpowers by some kind of scientific experiment or accident.
Ant-Man's power is in his suit. He's a normal person.
Superman is a Kryptonian (an alien.) He's not a mutant either; he's normal for a Kryptonian, it's just that being in the proximity of a yellow sun gives Kryptonians superpowers or that they get them when they mature (original origin story says the latter.)
Batman is another normal person with money and gadgets. He has no superpowers. He's the DC equivalent of Iron Man in that respect. Not a mutant.
The Flash typically gained his powers through a chemical accident, and was thus not a mutant.
Within Marvel, the term of "mutants" specifically refers to recipients of the "X-gene", homo superior. It is a specific genetic mutation with a wide variety of effects, occasionally indicated to be the "next stage" of human development. As Phyneas indicated, it's not just the X-men — the Morlocks, the Brotherhood of Mutants, and other X-gene carriers are included and there are non-mutant V-men.
Within the Ultimate Marvel Universe, "mutants" are the result of the Super-Soldier/Weapon X project, but they are similarly considered to be something different from people who are born with certain physical differences, such as Iron Man (he was born with neural tissue throughout his body), modified by treatments like Captain America (empowered by the Super-Soldier serum), or aided by technological means like Ant Man (gains his powers through the technical discovery of Pym Particles, although his wife, Wasp, is later revealed to be a mutant despite similar powers) or, again, Iron Man.
The existing answers cover Marvel well, but don't mention DC.
DC does have characters who would be considered mutants - people who have natural powers due to their genes, as opposed to people who gained powers through some sort of accident. The one that stands out in my mind is Jericho (of New Teen Titans fame). Danny Chase (also a Titan, from The New Titans), who had telekinetic abilities, also seemed to have them due to genetic factors, rather than some accident.
In 1988 and 1989, DC published a three-part mini-series, Invasion!, where a number of alien races tried to invade and conquer Earth. In the course of this series, we discovered that most (possibly all) of the humans who had acquired super-powers due to accidents have what is called a meta-gene. This appears to be very similar to the "x-gene" mentioned in Marvel's comics; in fact, Marvel has had mutants whose x-gene lay dormant, so they didn't gain powers normally (Lorna Dane, a.k.a. Polaris, is one of these).
During the Invasion storyline, some characters gained powers
DC's populace of people with the meta-gene seems to remain dormant far more often than Marvel's mutant population. Most "metahumans" gain their powers in association with some sort of accident, rather than having them simply appear due to the stress of puberty (the normal process for a Marvel mutant).
DC may not prominently refer to any of their characters who do seem to have powers not related to an accident as mutants because:
- The term "mutant" is so strongly associated with Marvel; or
- Because Marvel has the term trademarked (well, 'The New Mutants" is trademarked at least, and re-used often enough to maintain the trademark, I believe).
After all, the guy who says "Shazam!" was published under titles involving his magic word instead of his name (Captain Marvel) for several decades because Marvel had a trademark on "Captain Marvel"; to the point where DC changed his name to "Shazam", at least for a while.
I don't think I've seen the "meta-gene" explicitly referred to in more than 10 years. In the post-Flashpoint world, it's unclear if all powered humans (whether due to an accident, or to "natural causes") have the meta-gene as a common explanation or not.
(OK, I've now seen it in Deathstroke. However, it's possible the writer (Christopher Priest) included it based on his historical knowledge of the DCU, and the editor simply left it in place.)