Define the principal axis of a lens.

Hint : A lens is a transparent optical device that focuses or disperses a light ray through the phenomena of refraction. A simple lens is made of a single unit of transparent material. In general the principal axis joins the principal focus and center of curvature whether the optical instrument is a lens or a mirror.

Complete step by step answer
The principal axis of a lens is the line that passes through the geometrical center of the lens and joins the two centers of curvature of its surfaces. Sometimes the principal axis of a lens is called its optical axis. It also passes through the optical center and principal focus.

Additional Information
The optical center is the central point of a lens. A ray passing through the optical center of a lens travels without any deviation. The radius of curvature is the distance between the optical center and the center of curvature of the lens. Lenses are made out of various materials such as glass, plastic, etc.
The lens is ground and polished or cast into the desired shape to make the lens of various properties, shapes, and sizes. The Centre of curvature is the center of the sphere of which the lens is a part.

Note
An axis of a lens is a straight line passing through the optical center of the lens. So one must not get confused between the principal axis and axis of a lens. There are various sorts of lenses which are biconvex, plano-convex, biconcave, plano-concave, positive meniscus, negative meniscus. A ray of light parallel to the principal axis gets focused on the principal focus of the lens.