Omnipotent means able to do everything: power with no limit. It is mainly an adjective, the sister of the noun omnipotence. People say God is omnipotent. You will also see the Omnipotent, with a capital letter, used as a noun for God. That use feels old and church-like. Though actually, in ordinary English the word is often looser, and sometimes a little bitter. “She is omnipotent in that office” does not mean she can do magic. It means she decides everything, and nobody can push back. Younger speakers often pick the easier all-powerful; omnipotent sounds like a book, a sermon, or a newspaper trying to be grand. It is a strong, formal word. Do not use it for someone who is only quite strong or quite important. The three old “omni-” God-words — omnipotent, omniscient, omnipresent — are easy to mix: this one is about power, not knowledge, and not being in every place.