xcyclopswasrightx is close: the term Omega Level Mutant was first used in Uncanny X-Men #208 (August 1986), but was unexplained (aside from implying a staggering level of power), and not used again until 2001’s X-Men Forever.
No concrete definition has been given for what makes a Mutant an Omega-level Mutant (or any of the other levels; Alpha, Beta, Gamma, Delta, Epsilon, and Zeta have also been used), but common abilities include immortality (or something like it), extreme manipulation of matter and/or energy and/or minds, and the potential to exist in a non-physical form. So far twelve Omega-Level Mutants have been identified: Elixir (Josh Foley), Jean Grey, Nathaniel Grey, Hyperstorm (Jonathan Richards), Iceman (Bobby Drake), Legion (David Haller), Absolon Mercator, Quentin Quire, Franklin Richards, Hope Summers, Rachel Summers, Vulcan (Gabriel Summers). Additionally, Emma Frost and Charles Xavier have both been referred to as Omega-level telepaths. Despite his great power, Magneto has never been classified as an Omega-level Mutant.
The fact Jean Grey was/is an Omega-level Mutant is one thing that drew the Phoenix Force to her in the first place. Though this is a retcon since the whole Phoenix/Dark Phoenix thing was from 1976-1980, six years before the term “Omega-level Mutant” was ever first uttered. (But then lots of things about the Phoenix Force have been retconned over the decades.)