When biologists talk about a cline, they're referring to a variation in the characteristics of a species that is occurring across a population. These variations can occur to either genotypes or phenotypes, but to be considered a cline the changes must be happening in more than one environment or habitat.

Or, in plain English: when a species lives over an area, and there are changes in the environment to part (but not all) of that area, and the affected animal or plant population adapts to the change, we can measure the gradients or degrees of adaptations. That's a cline: a representation of the varying degrees of change.

Scientists usually specify what type of change has occured, be it geographic or ecological, with the terms geocline and ecocline, respectively.