Power 5 was what emerged from "BCS conference" when there were six conferences with BCS bowl autobids, and now there are five with New Year's Six bowl bids. It was never an official term; it's just what the media came up with based on perception.
No one knows what the bowl/playoff landscape will look like moving forward so it's not really clear what people will call the top conferences, or if the term will include the Big 12. If we get a 12-team playoff with 6 spots reserved for conference champs, then the Power Five leagues will make it basically every year and the 6th spot will go to an American/Mtn West/Sun Belt team, so I think the Power Five term would have a pretty good chance of surviving to refer to "the conferences that basically always make the playoff."
I think that having multiple teams credibly capable of being included in a 12 team playoff will be important if that's the direction the sport decides to go. You don't want to be a one bid league if every bid is worth $40 million to the conferences.
Imagine ISU gets one slot in this hypothetical playoff and the SEC gets four. ISU's cut from the Big 12, assuming 10 teams, is $4 million. Vanderbilt's would be $10 million for sucking hind tit and being the conference whipping boy. And let's face it, Big 12 teams will need that playoff money because our TV deal will be static at best. The 12 team playoff is why OU and UT will leave even though they risk their big dog status.
If it is an 8 team playoff, ISU might get $3-3.5M for their cut, but if conferences get a max of 2 teams in it, a 16 team SEC might only receive $3.75-4.3M after the split. Much more equitable, but it removes much of the advantages of consolidating the top brands into one super conference built to suck up all the tv dollars out there, which is definitely something the Big 12 and the Alliance say they want.