What is a figure having a polygon base and triangles sides which meet in a point at the top?

The figure you are describing is known as a pyramid. A pyramid has a polygonal base, and its triangular faces converge at a single apex or vertex at the top. The type of pyramid is often named after the shape of its base, such as a triangular pyramid (tetrahedron) or a square pyramid. Pyramids are three-dimensional geometric shapes commonly studied in geometry.