Visleaf
A visleaf, or merely a leaf, is one of many visleaves —think of them as sections— in which a map is divided during the compiling process. Each leaf is a volume that contains a section of geometry that is drawn together. When any part of a leaf is visible from the current leaf, all of the geometry in the leaf is considered for rendering by the engine. Calculating which leaves can see which is a large portion of [[|Vvis|VVIS]]' purpose.
You can view the current leaf in the engine by typing mat_leafvis 1 at the developer console. A red wireframe box will be drawn showing the current leaf. As you move around in the level, the box will redraw each time a new leaf is entered. Another way of visualizing visleaves is to use the glview (glview.exe) command-line tool.