"Homo sapiens mutatis mutandis" is the scientific classification for mutants in the Joint Venture Universe.
Homo sapiens, the classification for modern humans, translates to "wise man." Homo sapiens mutatis mutandis, therefore, translates into "wise man [with] the necessary changes having been made."
Professor Charles Xavier and Dr. Moira Kinross coined this term in the late 1980s, shortly after Xavier revealed his own mutant powers to Moira. Mutants had existed prior to that, going as far back as ancient Egypt and the Roman Empire, but were largely misunderstood by the scientific community, and humanity as a whole, and thus were referred to in any number of derogatory fashions. The most popular of these terms was "Witchbreed", a phrase that had spread through most of Europe and the New World.
By studying the history of the "Witchbreed", and any and all documentation about the history of the people who had been marked as such, Xavier and Kinross came to the conclusion that human mutation was both genetic in origin, and to a lesser degree, evolved in reaction to the mutant's environment. Xavier and Kinross formulated the theory of self-actualizing homomorphism, called "the Xavier-Kinross theory" for short.
The theory went that while the X-Factor in a potential mutant's genes was malleable to an extent, with any number of environmental or biological factors influencing the form a mutation would take upon activation, once activated, mutations would be hard-wired into a subject's DNA, with the mutation "fixed" and unable to be removed or altered in any significant way.
This theory was later proven conclusively by Dr. Linda Donaldson and her assistant, graduate student Hank McCoy, when the latter devised a serum designed to temporarily mimic the process of genetic mutation and exposed voluntary test subjects to repeated dosings. The test subjects' mutations activated in the same way each time -- a young man who gained reptilian scales and snake fangs would develop them every time he took the serum, with only slight variations (the color and pattern of his scales, the amount of venom stored in his fangs, etc,) each time.
Therefore, it was concluded that the "X-Factor" in mutant genes would determine what form a mutation should take upon first manifesting, and once chosen, the mutation would manifest in the same way every time, with the "X-Factor" having determined that the necessary changes had been made. Hence, the term Homo sapiens mutatis mutandis was born.
Homo sapiens mutatis mutandis vs. Homo sapiens superior
The most outspoken critics of the Xavier-Kinross theory came, not from baseline humans or even the scientific community, but from other mutants.
Believing themselves to be the next stage in human evolution, and thus "better" than humanity in some way, certain mutant radicals (like the terrorist organization Factor Six, or its successor, the Brotherhood of Mutant LIberation) preferred to describe mutants as Homo sapiens superior, or simply Homo superior. This term became inflammatory, sparking fear and hatred in a public that was already nervous around mutants to begin with.
Dr. Moira Kinross herself addressed these concerns at a Global Medicine Summit in Geneva. Mutants, she argued, had been around since the dawn of humanity. Had they truly been the next step in evolution, they woudl have outnumbered humans long ago, eventually replacing them completely. And while history did indeed show several significant "booms" in the mutant population, there were also long periods of time where no mutants at all were born, or very few. Kinross (and her colleague, Professor Xavier) would go on record as stating that she believed Homo sapiens and mutantkind were meant to live side-by-side, and that mutants were simply another form of humanity, having made necessary canges to adapt to their environment. While the general public remains divided regarding their opinion of mutants, the scientific community at least recognized Kinross and Xavier's logic as sound, and so Homo sapiens mutatis mutandis was considered the appropriate scientific term.
It is unknown of a true Homo sapiens superior species will emerge, either from humans or mutants. The closest thing to a true Homo superior that science officially recognizes is Steven Grant Rogers (AKA Captain America), and that was only achieved with the use of Dr. Abraham Erskine's "super soldier serum." Science, technology, and medicine might be able to create examples of Homo superior, but the odds of it appearing naturally are astronomical; however, to quote Anthony Stark, "the impossible is just something we haven't worked out how to do yet."