Foremost means most important, leading, or first in position or rank. When someone is described as the foremost expert on dinosaurs, they're recognized as the top authority, the person who knows more than anyone else in their field. When a problem is foremost in your mind, it's the main thing you're thinking about, overshadowing everything else.

The word combines two ideas: being first and being most significant. A country's foremost priority might be keeping its citizens safe. A scientist working on a cure for a disease keeps that goal foremost in her thoughts as she works late into the night. Your foremost concern before a big test is probably making sure you understand the material.

You'll often see foremost paired with “first,” as in “first and foremost,” which emphasizes that something is the top priority above all else. When a teacher says “First and foremost, everyone needs to be respectful,” she's making clear that respect matters more than anything else in her classroom. The word suggests not just importance, but being at the very front of the line in terms of significance or excellence.