General design principle to assist functionality but be out of your way

I don't have the language to even ask this question appropriately. I am designing a spec for a design of a UI that will assist the functionality of another device. I'd like to state in words that a design goal of this application in general is to be absent, that it should assist what the user wants to do, but not get in the way, the user shouldn't have to focus on this UI to have his work done. I don't even know where to start to put this in words though.

Good examples that I can think of are all kind of applications that assist with driving a car.