One of any number of PatternForms. The dominant form on Wiki is ThereforeBut. And click on TheReformSociety glyph at the bottom of this here page for an interesting extension of that.
A PatternForm is the literary structure of a written description of a pattern. The simplest pattern form is ThereforeBut. The DesignPatternsBook uses a more detailed and structured form. It is interesting to study the pattern form (as opposed to only studying patterns themselves) since the pattern form is a pedagological tool applicable to more than just software and architecture. In other words, "What makes patterns so powerful as a teaching and communicating tool?"