Retrogression, n.
Etymology: [Cf. F. rétrogression. See Retrograde, and cf. Digression.]
1. The act of retrograding, or going backward; retrogradation.
2. (Biol.)
Backward development; a passing from a higher to a lower state of organization or structure, as when an animal, approaching maturity, becomes less highly organized than would be expected from its earlier stages or known relationship. Called also retrograde development, and regressive metamorphism.
Shades of meaning
Fernald lists retrogression only as an antonym, under progress.
He lists it without comment; the entries above explain its neighbors.
Synonyms
none given