A Shear is a
transformation in which all the points move parallel to a fixed
line or plane, in such a way that the distance a point moves is
proportional to its distance from that line or plane.

For example,
if a pack of cards or pile of books on a table is pushed so that
the cross-section of the pile changes from a rectangle to a parallelogram
then the pile will have undergone a shear. With a shear the area
of a shape status the same.