Definition: The bottom of a shape, solid or three dimensional object. The base is what the object 'rests' on. Base is used in polygons, shapes and solids. The base is used as a reference side for other measurements, most often used in triangles. The base is the surface of the object that is stands on or it is the bottom line.
The bottom of the triangular based prism is considered the base. The bottom line of the trapezoid can be considered the base.