Definition
A Pattern Language is understood as " an organized and coherent set of patterns, each of which describes a problem and the core of a solution that can be used in many ways within a specific field of expertise." Source 
.
Explanation
Although speaking in the context of architecture and building, Alexander's work on pattern language has been applied and is applicable to a wide variety of design problems. See also Christopher Alexander, 1979. The Timeless Way of Building. New York: Oxford University Press.
Alexander speaks of Pattern Languages as sets of patterns that reflect knowledge of how to solve specific recurring problems.
See also: book Alexander, Christopher, 1977."A Pattern Language." Source