The underside of something is the bottom surface or the part that faces downward. When you flip over a leaf to look at its underside, you're examining the surface that normally faces toward the ground. The underside of a table is where you might find gum someone stuck there years ago, or where a carpenter's signature might be hidden.

Animals often have different colors or patterns on their undersides than on their tops. A fish might have a silvery white underside and a dark top, which helps camouflage it from predators looking up from below and from birds hunting from above. The underside of a cat's paws feels soft and padded, while the tops feel furry.

The word can also refer to something hidden or less obvious. When people talk about seeing the underside of a situation, they mean understanding the hidden, sometimes darker, parts that aren't immediately visible. A tourist might see only the beautiful buildings of a city, while residents know the underside: the problems, struggles, and challenges that exist beneath the surface.