All real materials contain defects that are departures from the ideal crystal structure. Defects are classified by their dimension:
 
 

Type	Dimensions	Characteristics
Point defects	zero dimension	Important in diffusion and other transport processes and can interact with dislocations (line defects) to change mechanical properties. Surfaces and internal boundaries provide easy paths for point defect motion.
Line Defects Dislocations	one dimension	Dislocations control the plastic deformation properties of crystalline materials. Their interactions with point defects, defect clusters, and internal surfaces may be exploited to produce materials with a range of properties.
Surface defects	two dimensions	Surfaces have different interatomic spacing than the bulk material and have an energy associated with them (surface tension). They are paths of more easy transport than the bulk, interact with dislocations, and through them impurities can enter or leave the material.
Volume defects	three dimensions	Voids in the structure, "negative crystals", second phase inclusions slag.