Underside

The underside is the side of an object, organism, or structure that is opposite the upper or visible exterior surface. It is often referred to as the bottom, ventral, or abaxial surface, depending on the field.

The term derives from under + side. In everyday language, underside implies the lower or hidden surface. Technical usage may prefer ventral, abaxial, or plantar terms to avoid ambiguity.

In botany, the underside of a leaf is the abaxial surface. Stomata are frequently more abundant on the abaxial face and play a key role in gas exchange and transpiration. The underside is often lighter in color and may bear hairs or wax.

In animals, the analogous term is ventral or belly; dorsal refers to the opposite back or upper side. The undersides may bear features such as sensory organs, scales, or ventral fins. In humans, the underside is the ventral aspect of the body.

In textiles, architecture, and engineering, the underside is sometimes treated differently for grip, insulation, or aesthetics. For example, roof tiles may have waterproofing or protective layers on the underside, and clothing linings lie on the underside. The word remains a general descriptor used across disciplines to distinguish the lower or hidden surface from the top or exposed side.