In his famous survey of mathematical ideas, Klein's Erlanger program, Felix Klein provided an organizing principle for geometry based on the notion of group of transformations and the study of its invariants.
“In classical geometry the primitive elements are points, and geometric objects are point sets with properties. The properties are of two main types: structural and transformational. Objects are characterized by structural relations and compared by transformations. In his Erlanger program, Felix Klein classified geometries by the transformations used to compare objects (for example, similarities, projectivities, affinities, etc). Geometric Algebra provides a unified algebraic framework for both kinds of properties and any kind of geometry.”
Gerald Sommer (Geometric Computations with Clifford Algebras)