For the purpose of this site, we follow architect Christopher Alexander, who launched Patterns and Pattern Languages as a way to develop and share knowledge around common design solutions for reoccurring problems in architecture.
Alexander understands patterns as a description of a recurrent problem in a specific environment and the core of the solution to that problem (OB paraphrasing "A Pattern Language" by Christopher Alexander et al., 1977. New York: Oxford University Press, page x).
Alexander speaks of Pattern Languages as sets of patterns that reflect knowledge of how to solve specific recurring problems.
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.
Understanding patterns helps us to understand systems. The understanding adds to Knowledge, which itself can be organized in patterns, for example in pattern languages.
Patterns are regularities in the organization and dynamics of systems. They can be intentionally created or be the emergent properties of Systems.