Wikipedia
In law, the ultimate fact is the conclusion (or conclusions) of factual evidence made by a jury after deliberation.
One common definition is "in a trial, a conclusion of fact which is logically deduced from evidence...." It is "the conclusion arrived during a trial based on the evidentiary facts."
An example would be the conclusion that somebody was liable for negligence because he sped, drove over a double line, skidded and lost control of his car. The jury can make out the ultimate fact when a man enters his apartment, a woman screams from inside, and the police find the man inside the apartment with his gun in his hand standing over his dead wife, that he murdered her.
A classic case, under New York law, is People v. Murphy. In that case, the defendant was rebuffed by his drug dealer's girlfriend and her sister, he invaded their apartment, sought and found a hammer, and hit each of three females (the pregnant girlfriend, her sister and her niece) with the claw end of the hammer, several times, on their heads. The ultimate fact decided by the jury was that he intended to kill all three of them. (He only succeeding in killing the girlfriend. The boyfriend returned from an overnight incarceration, rescuing the survivors. Their baby was delivered by caesarean section, the sister and her daughter also survived.)
Usage examples of "ultimate fact".
The ultimate fact is, the enemy was within your team's grasp, but they didn't succeed.
I was one, united to pure sensation, everything about me becoming a part of me, a fabulous unity, so that I knew at last the ultimate fact of alI existence, knew it and knew that there were no words with which it could be expressed because it was beyond words.
Certainly he was aware of many things that made it pleasant for him: the fact that he had the privilege, which he did not abuse, of taking time off whenever he wanted to, the fact that most of his time, when midday heat was at its worst, was spent in the air-conditioned retail shop, and the ultimate fact, from which these others derived, that he was the son of the owner.