【Deterioration】 , 【degeneration】 , 【devolution】 , 【decadence】 , 【decline】 ,  【declension】 are comparable as meaning either the process of falling from a higher to a lower level or the state of a thing when such a falling has occurred.

【Deterioration】 is the least specific of these terms and applies to any process or condition in which there are signs of impairment in quality, in character, or in value.

【Degeneration】 usually implies retrogression and a return to a simpler or more primitive state or condition; when used in reference to plants, animals, or their parts, it usually suggests changes in physical structure, but it may imply a progressive 【deterioration】  in structure and function resulting from disease.

When applied to persons in groups or as individuals or to states or empires, it suggests physical, intellectual, and often moral degradation and a reversion toward barbarism or, in the case of individuals, bestiality.

【Devolution】 in technical use may take the place of 【degeneration】 , but in general use it carries even a stronger implication of opposition to evolution.

【Decadence】 presupposes a previous maturing and usually a high degree of excellence; it implies that the falling takes place after a thing (as a people, a literature or other form of art, or a branch of knowledge) has reached the peak of its development.

In reference to matters of art 【decadence】 may imply no more than excessive refinement and studied attention to esthetic detail.

【Decline】 is often interchangeable with 【decadence】 because it, too, suggests a falling after the peak has been reached in power, prosperity, excellence, or achievement, but it usually suggests more momentum, more obvious evidences of 【deterioration】 , and less hope of a return to the earlier state.

【Declension】 differs from  【decline】 only in connoting less precipitancy or a slower or more gradual falling toward extinction or destruction.