Definition of "wellhole"

wellhole

noun
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.
  Quotations
  well-holes of the stairs and chimney
  1846, W. M. Buchanan, The Dictionary of Science and Technical Terms Used in Philosophy, Literature...
3. A cavity which receives a counterbalancing weight in certain mechanical contrivances, and is adapted also for other purposes.