A pattern description is a document that describes a pattern by giving at least the following information:
Besides the parts mentioned above, a pattern description typically also gives a motivation, some example code, possible variations of the pattern, known uses, etc.
The distinction between pattern and pattern description makes it clear that patterns are there—whether they are described or not. They are simply recurring solutions, that have to be described in order to communicate them and to make them reusable. This also means that patterns normally are not constructed but discovered. Someone realizes that a recurring problem has been solved basically in the same way over and over again. And after the pattern has been found, it can be documented using a pattern description. For convenience a pattern description is often called “pattern” too. But actually it is more than that.
See the list of documented patterns in this wiki.