Outer

Outer is a relative directional term used to denote something located on or near the exterior of an object, in contrast with its interior. In ordinary speech it can indicate the outer surface, edge, or boundary—anything farther from the center or core. The word appears across disciplines, from everyday descriptions of clothing or architecture to formal scientific or mathematical terminology, where precise definitions may vary by field.

In space and planetary science, 'outer' is used to differentiate regions and bodies. Outer space refers to the expanse beyond Earth's atmosphere. 'Outer planets' generally denote the gas giants beyond the asteroid belt, contrasted with the 'inner planets' of the terrestrial group. The term also labels external layers of a body, such as the outer crust or outer core of a planet.

In mathematics and computing, 'outer' appears in several standard constructions. The outer product of two vectors yields a matrix, and the outer measure extends a measure to all subsets. In databases, an outer join is a type of join that preserves rows with unmatched keys from one or both tables.

In everyday language, 'outer' is commonly used to describe clothing designed to be worn over other garments, known as outerwear, or outer layers of a structure, such as the exterior walls of a building. The term thus serves as a broad descriptor for anything situated on the outside relative to a reference point.