Retrogression
Part of speech: noun
Definition: A deterioration or decline to a previous state A return to a less complex condition
Degeneration
Part of speech: noun
Definition: The act or state of growing worse, or the state of having become worse; decline; degradation; debasement; degeneracy; deterioration.That condition of a tissue or an organ in which its vitality has become either diminished or perverted; a substitution of a lower for a higher form of structure.A gradual deterioration, from natural causes, of any class of animals or plants or any particular organ or organs; hereditary degradation of type.The thing degenerated.