Crystal Defect
Any irregularity that interrupts the perfectly ordered arrangement of particles in a crystal.
Unit
Dimensionless
In depth
Defects are thermodynamically inevitable above absolute zero because they increase entropy. They are classified as point, line, planar or bulk defects, and although they are imperfections, they are responsible for most useful properties — ionic conductivity, colour, mechanical strength and semiconductor behaviour all depend on them.
Examples in the real world
Dislocations letting metals deform plastically; F-centres colouring NaCl yellow; dopant atoms making silicon conduct.