1. A discontinuity in a crystal lattice. A point defect consists either of a missing atom or ion creating a vacancy in the lattice (a vacancy is sometimes called a Schottky defect) or an extra atom or ion between two normal lattice points creating an interstitial. A Frenkel defect consists of a vacancy in which the missing atom or ion has moved to an interstitial position. If more than one adjacent point defect occurs in a crystal there may be a slip along a surface causing a line defect (or dislocation). Defects are caused by strain or, in some cases, by irradiation. All crystalline solids contain an equilibrium number of point defects above absolute zero; this number increases with temperature. See feature at crystal defect.