A straight line about which a surface or solid revolves.
The axis of revolution of a right circular cylinder is a straight line passing through the centres of its congruent bases. This axis is perpendicular to the bases.
The axis of revolution of a right circular cone is the line that passes through the apex of the cone and through the centre of its circular base. The axis is perpendicular to the base.
The axis of revolution of a sphere is the diameter of the sphere.
Right circular cylinder and its axis of revolution: