Uppermost means highest in position, or most important in your mind. When you stack books on a shelf, the uppermost book sits on top of all the others. When a thought is uppermost in your mind, it's the one you're thinking about most, crowding out everything else.

You'll see this word used in both physical and mental ways. A mountain climber might reach the uppermost peak of a range, standing higher than any other point around. But a student worried about an upcoming presentation might say that concern is uppermost in her thoughts, meaning she can't stop thinking about it even when she tries to focus on other things.

The word carries a sense of supremacy or dominance. The uppermost branches of a tree catch the most sunlight. The uppermost priority in an emergency is keeping people safe. When something is uppermost, nothing rises above it, whether we're talking about height, importance, or what's occupying your attention at a given moment.