"K" is an imaginary plane which divides a crystal into two equal halves which are the exact mirror images of each other. The proper term for "X" is

"K" is an imaginary plane which divides a crystal into two equal halves which are the exact mirror images of each other. The proper term for "X" is

Explanation

 The proper term for "X" is Plane of symmetry.

A plane of symmetry is an imaginary plane that divides a crystal into two halves, where each half is the mirror image of the other.