Science, Tech, Math › Math Definition of a Base Share Flipboard Email Print Base of Triangle and Trapezoid. D.Russell Science, Tech, Math Math Tutorials Geometry Arithmetic Pre Algebra & Algebra Statistics Exponential Decay Functions Worksheets By Grade Resources View More By Deb Russell Math Expert Deb Russell is a school principal and teacher with over 25 years of experience teaching mathematics at all levels. our editorial process Deb Russell Updated March 06, 2017 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. Examples: The bottom of the triangular based prism is considered the base. The bottom line of the trapezoid can be considered the base.