Space diagonal

The term "space diagonal" refers to the diagonal line that connects two opposite corners of a three-dimensional geometric shape, such as a cube or a rectangular prism. Unlike face diagonals, which are diagonals that lie on the faces of the shape (two-dimensional), space diagonals extend through the interior of the shape. For example, in a cube, a space diagonal connects one vertex (corner) of the cube to the opposite vertex that is farthest away.