In population genetics, a cline is a gradual change of a character or feature (phenotype) in a species over a geographical area, often as a result of environmental heterogeneity. The change in phenotype does not result in different species as long as the geographically spread populations can interbreed with one another. This meaning of cline was introduced by Sir Julian Huxley.

A ring species is a special kind of cline where the geographical distribution in question is circular in shape, so that the two ends of the cline overlap with one another, giving two adjacent populations that rarely interbreed due to the cumulative effect of the many changes in phenotype along the cline. The populations elsewhere along the cline interbreed with their geographically adjacent populations as in a standard cline.

A "cultural cline" is a gradual change of a cultural characteristic or feature over a geographical area. Source of definition unknown.

In linguistics, the term cline refers to a list of words dealing with the same concept, arranged in order of intensity. For example a list could be made with words like "freezing", "cold", "cool", "tepid", "warm", "hot", and "scorching".

From Greek ^*, "to lean"

Rejection of "Race" for Cline

Because the term race has often been used synonymously with subspecies, some people incorrectly use cline instead of "race" to describe what they see as distinct groups of humanity (Caucasoid, Mongoloid, Negroid, Australoid, Capoid, and so forth). This substitution of "cline" for "race" or "subspecies" is technically incorrect because the word "cline" refers only to the geographic density gradient of a single feature, while the words "race," "subspecies," etc., assume replicable clusters of features. On the other hand, using the term "cline" in substitution for "race" may be appropriate if the speaker understands racial distinctions more in terms of groupable variations on a continuum. In line with the example above from linguistics, human skin color may be described in terms of the following cline: "white", "cream","olive", "tan", "brown", "black", and etcetera with a myriad intermediates. Because the controversies over the term "race" the term "cline" may be much more of an accurate description. See the main article Race for details on this point.

Population genetics