Question

define the term focus of convex lens

Answer 1

focus is a point where all parallel lights to principal axis  reflect by convex lens    

and meet at same point and image formed

Answer 2

Canvas lens thicker in middle when parallel rays of light pass throw a convex lens the refracted rays convert one point called the principal focus. The distance between the principal focus and the centre of the lens is called the focal length.

Hope it will helps you.