The outermost thing is the one farthest from the middle. Think of rings: the outermost ring is the last one. Planets, layers of skin, walls of a city, seats in a theatre — if you can point to a centre, this word points to the far edge. It is a calm, careful word; science and maps like it. “Outer” already means “on the outside”; “-most” pushes it to the extreme. Though actually, outermost and uttermost began as twins of the same old word. Today we mostly keep them apart: outermost is about place (farthest out), and uttermost is more about degree (the very greatest). Some older writing still mixes them.