From Valve Developer Community
BSP stands for Binary Space Partition. *.bsp is the file extension for maps/levels used by BSP based game engines. For more information how BSP works see the Wikipedia article on binary space partitioning.
Note:The BSP file name must not exceed 30 characters, with the ".bsp" extension included.
- Quake 2 Engine BSP format (for understanding and comparison).
- Information at Wikipedia about Binary space partitioning.