Variation
in astronomy, any deviation from the mean orbit or mean motion of a heavenly body produced by the perturbation of another body or bodies. Thus the planets are considered to move mathematically in elliptic orbits, which would be the case if they were subject to the attraction of the sun only, but being acted on by each other, there is supposed to be a minute and slow but constant variation in the elements of the ellipse. Variations which are compensated in short intervals are called periodic, and those which require for their compensation a long period are called secular.
In biology: (1) A tendency in all organisms to vary slightly from other organisms produced by the same parents. (2) Hereditary modification. (3) A modification directly due to the physical conditions of life; such as the dwarfed condition of shells in the Baltic, or of stunted plants on Alpine summits. (4) An organism, or a group of organisms, exhibiting modification due to external conditions.