World Brush
A world brush is a brush that has not been tied to an entity. Unless specified otherwise by a material, world brushes are solid to everything, block line of sight, cast lightmap shadows, cut visleaves, obstruct visibility, and seal the map. New world brushes are created with the Block tool 
. To turn a brush entity into a world brush, select it, then choose Tools > Move to World (or press Ctrl+⇧ Shift+W).
Risk of Confusion:Brushes tied to func_detail and func_group are technically still world brushes, as they are added to worldspawn in the compiled BSP. These are used by the compiler to know how to handle specific world brushes (such as skipping VIS for all func_detail brushes).func_ladder is a regular brush entity in 
GoldSrc, but is converted to a world brush in pre-
versions of 
Source.