wellhole

Noun

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