It can be a sphere, a spheroid, an ellipsoid, etc.
It is called a face.
a face.
cone
The answer is a cone
The sphere.
a face
A conic section.
a pyramid
A polygon is a many sided 2-dimensional figure, and thus could never be "solid" Alternatively, it could be "one face that is a polygon". In that case the solid could be like a pyramid that morphs into a cone or a part-sphere. The base would be the one polygon, the other face would not.
A square pyramid
A solid figure with 8 faces is called an octahedron. In a regular octahedron, each face is an equilateral triangle, and it has 6 vertices and 12 edges. Octahedra are one of the five Platonic solids and can be found in various applications, including geometry and crystallography.
A cone, possibly.