1) Transition from higher forms of development to lower ones, movement backwards, change for the worse; a concept opposite to progress.
2) In biology – the simplification of the structure of the organism in the course of evolution due to adaptation to changed conditions of existence.
3) In rehabilitation and defectology – the degradation of a certain function, its return to a lower level of development. It can be both temporary (caused, for example, by a somatic illness at an early age) and persistent (associated with serious damage to function, for example, a return to autonomous speech in autism spectrum disorders or to childhood forms of behavior in hebephrenia). Less mature functions are subject to regression. It must be distinguished from disintegration, the gross disorganization of a function.