wellhole

From well +‎ hole.

wellhole (plural wellholes)

1. The open space in a floor, to accommodate a staircase.
2. The open space left beyond the ends of the steps of a staircase.
   • 1846, W. M. Buchanan, The Dictionary of Science and Technical Terms Used in Philosophy, Literature...:

     well-holes of the stairs and chimney

3. A cavity which receives a counterbalancing weight in certain mechanical contrivances, and is adapted also for other purposes.

“wellhole”, in Webster’s Revised Unabridged Dictionary, Springfield, Mass.: G. & C. Merriam, 1913, →OCLC.