The base of a pyramid can be trilateral, quadrilateral, or any polygon shape, meaning that a pyramid has at least three outer triangular surfaces (at least four faces including the base).
The base of a pyramid can be trilateral. This is quadrilateral. The base of a pyramid can be any polygon shape. This is quadrilateral. A pyramid has at least three outer triangular surfaces (at least four faces including the base). This is what the base of a pyramid is meaning.