Per Mike Aresco, it is a media-created term.
From his Open Letter on Power 5 - Group of 5 Branding:
"Power 5 is a media-created term".
"The compelling issue at hand is the manufactured P5 label and the attendant fallout that is damaging to college athletics. It is the corresponding media-driven P5 designation that has consigned over half of FBS football to a perceived second-class status".
"The media actually coined the term to replace the former Bowl Championship Series (BCS) description, and it often made little or no sense even during the last decade. The harmful divide that has developed can trace its roots to the advent of the BCS in 1998, which designated selected conferences as annual participants in BCS bowl games. The divide accelerated with the advent of the College Football Playoff, wherein the BCS label morphed into the media-invented P5. This divide did not exist in any meaningful way back in the College Football Association days of the 1980’s and 1990’s. No divisive nomenclature and arbitrary classification existed back then".
"Whenever a so-called non-P5 school wins the national championship in men’s basketball, as the American did in 2014, three national championships in women’s basketball, as the American did in 2014, 2015 and 2016, or makes the College Football Playoff, as the American did in 2021, or in addition makes (seven times) and wins (four times) a New Year’s Day Bowl game against a top ten team or makes the men’s Final Four and the Elite Eight and the Sweet Sixteen, how are those teams “non-power”? It is absurd, and proof that the power moniker makes no sense competitively. The fact that so-called P5 teams that have not achieved at that level are still deemed “power” teams is an absurdity on its face. Another absurdity involves realignment, where Group of Five teams instantly become Power Five teams simply by signing a piece of paper".
by Commissioner Mike Aresco
theamerican.org