(1) An expression of an observed regularity. (Alexander 1979)
(2) A representation of similarities in a set or class of problems, solutions, or systems. (Alexander 1979)
(3) Each pattern describes a problem which occurs over and over again in our environment, and then describes the core of the solution to that problem, in such a way that you can use this solution a million times over, without ever doing it the same way twice. (Alexander 1979)
(1) - (3) Alexander, C. 1979. The Timeless Way of Building. New York, NY, USA: Oxford University Press.
A full discussion of patterns and how they relate to systems thinking can be found in Patterns of Systems Thinking.