This is a sufficient definition and from human POV; that is, if something like what is described existed, we would call it omnipotent.
Let U be a closed universe, or something very near closed. Closed means that the things inside it can’t get to or sense the outside, and hence are unlikely to know anything about it. Let G be an undefined entity (you can read it as God if you really want to).
G is omnipotent with regards to U if G can shape U into whatever native form it could encompass. So, for example, in our universe an omnipotent G could create and remove physical objects at will, but it would not be necessary for G to be able to create things fundamentally beyond our understanding (I have a few problems with trying to create examples for certain reasons), because they are not part of our universe as is.
From this basis, a theorem: G must be outside the conception of time (or entropy or another measure of change, with apologies to everyone who knows physics for probably misusing “time” and “entropy”) that exists in U.
If this was not the case, G could first (within the dominant measure of change) create the indestructable wall and then create a cannonball that destroys everything it touches, third make them touch, which results in impossible outcome, and hence is not true. This does not happen when G is outside the conception of change as it applies in U, because then G would both create something and cancel it at the same time, which amounts to not creating the thing to start with, which leads to no paradox, because G didn’t actually create one of the conflicting absolutes after all.
The definition also assumes G is omniscient, but when talking about omnipotence, that is kinda trivial.