> 'Official act' does not currently have a legal definition. It isn't defined in this majority opinion and it hasn't been given a definition previously.

It sounds like it does, from the majority opinion:

> The President enjoys no immunity for his unofficial acts, and not everything the President does is official. The President is not above the law. But Congress may not criminalize the President’s conduct in carrying out the responsibilities of the Executive Branch under the Constitution. And the system of separated powers designed by the Framers has always demanded an energetic, independent Executive. The President therefore may not be prosecuted for exercising his core constitutional powers, and he is entitled, at a minimum, to a presumptive immunity from prosecution for all his official acts.

So it sounds like "official acts" means an act "exercising his core constitutional powers."

Those are two independent clauses which means that neither is a definition of the other.

You're misreading this.

Core constitutional powers: may not be prosecuted. Period. Impossible.

Official acts: presumptive immunity, may possibly be prosecuted.

Clearly "core constitutional powers" != "official acts" because they have two very different standards applied.