Question

what is meant by linear magnifcation of a concave mirror

Answer 1

The linear magnification or magnification of a spherical mirror may be defined as the ratio of the size (height) of the image to the size (height) of the object. The magnification of a mirror is represented by the letter m.

Answer 2

Linear magnification produced by a spherical mirror is the ratio of the size of the image formed by the mirror so that of the size of the object. It is also formed as negative of the ratio of the image distance so that of the object distance. If m is the magnification, O the size of object, I the size of image, u and v are the respective distances of the object and the image from the pole of the mirror, then 
m = I/O = -v/u
The expression is same for both concave and convex mirrors.