Solution.

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.

The pattern concept can usefully be contrasted with the idea of a rule. A rule indicates what is to be done in a given situation. A pattern raises issues that may need attention and provides ideas and examples of what could be done to address them. Thus, while a rule bypasses people’s intu­itions, a pattern calls upon these intuitions and attempts to educate and strengthen them in the process of solving the problem. 

Christopher Alexander, from A Pattern Language