In architecture, a knee wall is typically a short wall

Wall

A wall is a usually solid structure that defines and sometimes protects an area. Most commonly, a wall delineates a building and supports its superstructure, separates space in buildings into rooms, or protects or delineates a space in the open air...

, usually under three feet (one metre) in height. In his book A Visual Dictionary of Architecture, Francis D. K. Ching

Frank Ching

Frank Ching is a widely recognized author of books addressing architectural and design graphics. Ching's books have been widely influential and continue to shape the visual language of all fields of design. He is a Professor Emeritus at the University of Washington.Ching was born and raised in...

defines a Knee Wall as "A short wall supporting rafters at some intermediate position along their length." The term is derived from the association with the vertical location of the human knee. Knee walls are common in old houses that are typically not a full two storeys in height, in which the ceiling on the second floor (in the "attic" area) slopes down on one or more sides. These houses are sometimes referred to as one-and-a-half storeys.

It is also commonly known as a half-wall, buttress or partial-wall.