What is a pattern?

Q. What is a pattern?
A. In [Christopher] Alexander's own words, "... we may define a pattern as any general planning principle, which states a clear problem that may occur repeatedly in the environment, states the range of contexts in which this problem will occur, and gives the general features required by all buildings or plans which will solve this problem." (The Oregon Experiment, p. 101) These patterns ideally function together as words in a sentence, creating a cohesive whole built on a common design language, the pattern language.

 Source: http://uplan.uoregon.edu/faq/FAQPatternLanquage.html