A mesa is a flat-topped hill most commonly found in the Southwest part of the U.S.. Its sides are steep all around so that it looks like a massive table.
Mesa comes from the Latin mensa meaning "table," which is very much what a mesa looks like. A mesa is formed when the weaker horizontal rocks around a big formation start to erode and fall away, leaving stronger rocks standing in a flat-topped hill. Grand Mesa in Western Colorado is the largest mesa in the world, with an area of 500 square miles. That's bigger than all of Hong Kong!