The difference of dislocations from other defects in crystals is that in case of dislocations a significant distortion of the regular atomic arrangement is concentrated in a small area close to a line that runs through the crystal. Such distortions are linear phenomena such as the breaking or shearing of atomic layers disrupting their order of arrangement. The cross section dimensions of a linear defect do not exceed one or several atomic spacings and the length can reach the size of the crystal. The concept of dislocation was introduced by Frenkel and Taylor in 1940s to explain the mechanism of plastic deformation.

There are two types of dislocations: edge dislocation and screw dislocation. If one of the atomic planes in the crystal breaks, an edge dislocation occurs; the edge of the "extra" half-plane forms its axis. The simplest visual model of an edge dislocation is a book in which a part of one page is torn apart. Then, if the book's pages represent atomic planes, the edge of the torn page simulates the edge dislocation line. Screw dislocation occurs when the lattice of one part of the crystal is slipped across the other part by lattice constant. It occurs along a half-plane of atoms parallel to s line of dislocation , which plays the role of the dislocation axis.

Dislocations are always present in real crystals. The typical concentration of dislocations in metals is 10¹¹ m^-2, while for insulators it equals to 10⁸ m^-2. Dislocations are studied using the methods of selective chemical etching, photoelasticity analysis, X-ray diffraction analysis and electron microscopy.