On Monday, December 19, the election will be formalized and made official in the Electoral College. Trump is not actually President-Elect yet....he still has one more hurdle to overcome. Although, on November 8, he got more than the 270 needed Electoral votes on paper, the Electors themselves still have to actually vote (which they do on December 19). It is rare for committed Electors to defect from their stated candidate, but it has been known to happen. In 1968, a Nixon Elector defected to George Wallace, an ex-segregationist who had run that year as an Independent and won five Deep South states. That one defector, of course, wasn't enough to sway the election....which ended up 301 votes instead of 302. However, some of the Trump Electors this year will be under intense pressure from some of the media and some of Hillary's supporters to change their committed votes and not elect Trump. (to make it clear, I'm not saying I agree with this pressure, which I don't, but, in fact, it exists). Some state laws require the Electors to vote for their announced candidate....others do not bind them, and allow them to change their votes, or even vote for a write-in. What happens on the 19th will determine if Trump is actually President, or if the election is handed to someone else. In an extreme case, if no one gets 270 Electoral votes, or if it is tied at 269/269 (which is possible), then the House of Representatives would decide the election. Even with Republican control of the House, though, Trump's election still wouldn't be guaranteed......there are even some Republicans in the House who don't care for Trump, though my guess is they'd think twice before voting for Hillary.
So, if Trump gets past Monday's vote....he's in. But that vote is not a sure thing yet.