window_pane

This entity is not in the FGD by default.
It should not be put directly in a map.

Class hierarchy
CWindowPane ↳CBaseAnimating ↳CBaseEntity
func_breakablesurf.cpp

window_pane is a model entity available in all  Source games. It is a shard that falls out of a func_breakable_surf.

Half-Life 2 window pane example.