1. Reversion; retrogression.
2. Relapse to a less perfect or developed state.
3. Psychology. Reversion to an earlier or less mature pattern of feeling or behavior.
4. Medicine. A subsidence of the symptoms or process of a disease.
5. Biology. The return of a population to an earlier or less complex physical type in successive generations.
6. Statistics. The relationship between the mean value of a random variable and the corresponding values of one or more independent variables.
7. Astronomy. Retrograde motion of a celestial body.
8. Geology. A relative fall in sea level resulting in deposition of terrestrial strata over marine strata.

The American Heritage® Dictionary of the English Language, Fourth Edition Copyright © 2007, 2000 by Houghton Mifflin Company. Updated in 2009. Published by Houghton Mifflin Company. All rights reserved.