wellhole

from The Century Dictionary.

• noun A deep, narrow, perpendicular cavity, as the space from top to bottom of a house round which stairs turn; also, an inclosure in which a balancing-weight rises and falls, etc.
• noun The well-room of a boat.

from the GNU version of the Collaborative International Dictionary of English.

• noun The open space in a floor, to accommodate a staircase.
• noun The open space left beyond the ends of the steps of a staircase.
• noun A cavity which receives a counterbalancing weight in certain mechanical contrivances, and is adapted also for other purposes.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun The open space in a floor, to accommodate a staircase.
• noun The open space left beyond the ends of the steps of a staircase.
• noun A cavity which receives a counterbalancing weight in certain mechanical contrivances, and is adapted also for other purposes.