Must be a public, non-sarcastic apology for something he hasn't apologized for in the past.
In cases of ambiguity, this question uses definition 1a from Webster's Dictionary:
an admission of error or discourtesy accompanied by an expression of regret
Pleading guilty to a crime does not necessarily count as an apology, but there may be situations in which Trump does apologize for something in court and those would count even if he's just doing it for legal reasons.
These rules may be updated to better cover edge cases, so please bring up any you think of in the comments. I will not trade in this market, except to set the initial probability.
This market resolves to earliest time period in which Trump apologizes for something, or to "Not before Election Day" if he doesn't by then.
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@Joshua This is a brilliant corner case that I thought of with the original market but the criteria was already problematic enough. I would say there's two options in such a case: resolve to 50%, or wait for him to walk out of the courtroom where he'll surely recant the entire apology to the press.
@Pykess I prefer not to plan on any 50% resolution, I'd rather just count it if he is made to apologize for something in court unless anyone wants to argue that doesn't count. I think he was also basically forced to apologize for the Access Hollywood tape, but that was still an apology.
I'll note that a plea deal isn't an apology though. I'm thinking about cases where a judge threatens to hold him in contempt of court unless he apologizes to the court or something like that. Not sure how common that is, but that should count IMO.