A pyramid (from Greek "πυραμίς" – pyramis) is a structure in which all of the outer surfaces (excluding the base) are triangular and converge at a single point. The base of a pyramid can be trilateral, quadrilateral, or any polygon shape, meaning that a pyramid has at least three triangular surfaces (at least four faces including the base). The square pyramid, with square base and four triangular outer surfaces, is a common version. A pyramid's design, with the majority of the weight closer to the ground, and with the pyramidion on top, means that less material higher up on the pyramid will be pushing down from above: this distribution of weight allowed early civilizations to create stable monumental structures.

Source: princeton.edu