Yes, there seems to be a thing regarding what constitutes a "Portillo Moment", rather like the differing definitions of "jumping the shark".
As Rover pointed out (and as I recall) there was no inkling on election night that Portillo was going to lose. Nary a mention of a possible upset was put to him by Jeremy Paxman earlier on in the evening. So one marker of a so-called "Portillo Moment" is it's unexpectedness. Cross-referencing the discussion in the General Election thread, I remember poring over the nightly polls in the run-up to '97 (I was a university student then) to try to detect any late movement towards the Tories, haunted as I was by 1992 (and historically 1970) before I was satisfied that Labour was at least going to win.
The media were same - very cautious about the likely outcome (late-swing, shy Tories etc). It was only on the Sunday before Polling Day, when the Observer did their famous list of possible Tory high-profile defeats through a measure of tactical voting (inc. Portillo's "safe" seat of Enfield Southgate) that the media were starting to stick their neck out and propose a proper beating for the Tories was in store.
Now some have gone though the "it must be an ultra-high profile Tory" route for it to qualify as a "PM". However, although Portillo did have a higher profile than a lot of Tory ministers at the time, he was at the end of the day the Defence Secretary - not Chancellor, not Home Secretary, not Foreign Secretary. But he was seen as the future hope for the Tories (no guarantee he was going to win the ensuing leadership election even if he had hung on to his seat). It was what he represented - that preening Uber-Thatcherism, "Who Dares Wins" and all that and for him to be taken out of Parliament altogether was what was so satisfying. I watched the results with some mates and we were all drunk at 3 in the morning and when he lost we had a group hug!
It was the symbolism of it. In that sense you could apply the Portillo moment retrospectively (Benn losing in 1983, Shirley Williams in 1979), indeed George Brown at Belper in 1970 was perhaps the prototype Portillo Moment, as he was Deputy Leader of the Labour Party, former Foreign Secretary and one of the most high-profile figures in 1960s British politics - the face of Labour's catastrophic defeat at the hands of Ted Heath's Conservatives.