Point defects: Point defects are defects which are not extended in space in any dimension. There is no strict limit for how small a "point" defect should be, but typically the term is used to mean defects which include at most a few extra or missing atoms without an ordered structure of the defective positions. Larger defects in an ordered structure are usually considered dislocation loops. For historical reasons, lot os point defects especially in ionic crystals are known as 'centers': for example the vacancy in many ionic solids is known as an F-center..