Trump says he won't attend the inauguration.

Just before that tweet, there was this: And last night, this zombiesque performance: AND: Here's the transcript for that zombiesque performance. I saw some people calling this his "concession," but I listened and heard no concession: 
We have just been through an intense election and emotions are high, but now tempers must be cooled and calm restored. We must get on with the business of America. My campaign vigorously pursued every legal avenue to contest the election results. My only goal was to ensure the integrity of the vote.... Now Congress has certified the results. A new administration will be inaugurated on January 20th. My focus now turns to ensuring a smooth, orderly and seamless transition of power. This moment calls for healing and reconciliation.... We must revitalize the sacred bonds of love and loyalty that bind us together as one national family....

He's saying the process for challenging the result has concluded and he will not challenge it anymore. It's not that Biden really won, but that the process is really over. What exactly is "concession"? It's not a technical term. Nothing depends on it. It's a nicety. We want a particular locution... but why? Is it like saying "uncle"

From the Wikipedia article "Concession (politics)":

In modern U.S. elections (presidential or otherwise), a concession is usually a two-step process: first, the losing candidate makes a concession phone call to the winning candidate and congratulates them personally. Second, the losing candidate makes a televised public speech, known as a concession speech, to their supporters, on an (improvised) podium surrounded by the candidate for the vice presidency, their spouses or other important relatives and friends. The concession speech consists of four elements: 
1. The statement of defeat: an admission that the candidate has lost the election to their opponent, who is congratulated on their victory. 
2. The call to unite: an expression of support for the victor's upcoming term in office, and a call for unity under their leadership, necessary after an often divisive and polarizing election campaign. 
3. The celebration of democracy: a reflection on why democracy and the participation of millions of voters in the electoral process is important, and that their choice should be respected. 
4. The vow to continue the fight: a reminder of the importance of the issues the candidate has raised during the campaign, and the policies their party advocates for. The candidate says that these remain important goals to strive toward, promises to continue fighting for them, and urges their supporters to do the same...

Trump's speech does not have the most important element: The congratulation of the opponent! He never mentions Biden. He does not concede that Biden won, only that the process was conclusive. 

Elements 2 and 4 are there, but the 3rd element is reversed. He doesn't say that people voted and he must bow to the people's choice. He's keeping alive the belief that the result that has been certified is not what the qualified voters actually voted for. In his rhetoric, democracy does not prevail. The idea that supersedes democracy is order — smooth, seamless orderliness. 

BUT: Element 2 is incomplete. He wants the people to unite, but just in general. He doesn't say unite behind our new leader!