An underpart is the lower or bottom section of something, especially the underneath surface of an animal's body. When you see a robin hopping across your lawn, the bright orange feathers on its chest and belly are its underparts. Birds often have different coloring on their underparts than on their backs: a hawk might have dark brown feathers on top but pale, streaked underparts that help it blend in when hunting.

The word is especially useful for describing animals because the bottom and top of a creature often look quite different. A shark has a dark gray back but white underparts, which helps it hide from both prey swimming below (who see the light underparts against the bright surface) and predators above (who see the dark back against the deep ocean). Scientists studying wildlife pay close attention to underparts because these markings help identify different species.

You might also hear the word used for mechanical things: a mechanic inspects the underparts of a car, or an engineer examines the underparts of a bridge. In these cases, it simply means the parts or surfaces underneath the main structure.