Topmost means the highest or uppermost of all. The topmost branch of a tree stretches higher than any other branch, reaching toward the sky. The topmost floor of a skyscraper offers views that nobody below can see. If you stack books on a shelf, the topmost book sits on top of the entire pile.

On a climbing wall in gym class, the topmost handhold marks the finish. On a mountain, the topmost peak towers above all the surrounding summits.

You'll also see topmost used to mean most important or highest priority. When a detective investigates a mystery, finding the culprit becomes the topmost concern. A student preparing for finals might list math as her topmost priority if that's the test she needs to focus on first. In this sense, topmost describes something that ranks above everything else in importance, sitting at the top of an invisible list in your mind.