The American Heritage® Dictionary of the English Language, 4th Edition
n. A gradual change in a character or feature across the distributional range of a species or population, usually correlated with an environmental or geographic transition.
n. A gradation in a character or phenotype within a species or other group.
n. Any graduated continuum.
Word Usage
"Hate to muddy the waters, Jenny, but apparently it's not meaningful to give average weights because there is a "cline" in size from Spitzbergen, where the bears are smallest, to the Bering Strait where they are largest."