A diagonal is made out of a straight line that's set at an angle instead of straight up or across. If you picture a square and draw a line connecting the opposite corners, that’s a diagonal line.
You’ll find diagonal lines in geometry, and also in the world around you. A quarterback might throw a diagonal pass that angles across the field, or you might buy a snazzy new leotard with diagonal stripes. Diagonal can be either a noun or an adjective. You could call the steep slope of a hill a diagonal across the sky, or you could talk about the diagonal hillside reaching up toward the clouds.