If a person is reflected in a plane mirror, the image of his right hand appears to be the left hand of the image. This inversion of the left hand side of the object into the right hand side in the image and vice-versa is called lateral inversion. Image in a plane mirror is laterally inverted.
Cause of lateral inversion: In a plane mirror, the image is as far behind the mirror as the object is in front of it and that the front of the image and the front of the object face each other. This is the cause of lateral inversion in plane mirror.