A Brush Entity is an entity type in the GoldSource and Source Engines, created by tying an entity to a BSP geometry brush in the map, giving the brush a specific affect or ability defined by the entity tied to it.
Purpose and Uses
Brush entities are used for entities that require an area or trigger zone to be touched, hit, or used in order to perform a specific task. Another use of brush entities is for creating an entity that requires a model, a textured brush(or series of brushes) to create a physical object, such as an elevator, or a door. A great example of a brush entity that implements both of these designs is a func_button, which requires a brush to specify the area the player must press, touch, or shoot to activate the button; the button can be made using any shape of brush, or even a group of brushes, and be textured to give the appearance of a fancy button or even a lever or turning wheel. Brush entities such as the func_button have special features that affect the physical brush they're tied to, such as moving the brush in a general direction when activited, giving the affect of the button being pushed in, activated, and then popping back out. Brush entities are used primarily for triggering another object or event, or for developing moving brushes to make things like conveyor belts, trains, and ceiling fans.