General:
* Cavity: A hollow space within a solid object.
* Aperture: An opening, especially a small one.
* Orifice: An opening, especially one that is narrow.
* Void: A completely empty space.
* Pit: A deep hole in the ground.
* Crevice: A narrow crack or opening.
Specific:
* Pothole: A hole in a road.
* Tunnel: An underground passage.
* Den: A hole in the ground used as a shelter.
* Bore: A hole drilled into something.
* Cleft: A narrow, deep opening.
* Chasm: A deep, wide opening.
Figurative:
* Gap: A space or interval between things.
* Breach: A break or interruption.
* Flaw: A defect or imperfection.
* Shortcoming: A weakness or fault.
Choose the synonym that best fits the situation you are describing.