window_pane

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

class hierarchy
CWindowPane defined in func_breakablesurf.cpp ↳CBaseAnimating ↳CBaseEntity

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

Half-Life 2 window pane example.