underside

An underside refers to the bottom or back side of something, like an object or an animal. It's also known as the lower part.

When we talk about the underside, it usually means that we're looking at or referring to what's hidden from view, often because it's on the opposite side or below something else. For example, when you look at a bridge, you can see the upper part where people walk or drive, but the underside is the lower section that's attached to the ground. If you have a pet, like a dog or a cat, the underside usually means the belly or stomach area. In some cases, the word "underside" might be used for something that's not visible at all, like the inside of a machine or the bottom of an empty box.