Definition (Not to be confused with Room):
Room for a boat to leave a mark on the required side. Also,
(a) room to sail to the mark when her proper course is to sail close to it, and
(b) room to round or pass the mark as necessary to sail the course without touching the mark.
However, mark-room for a boat does not include room to tack unless she is overlapped inside and to windward of the boat required to give mark-room and she would be fetching the mark after her tack.
This definition changes in the 2021 Racing Rules of Sailing update. Read an analysis of changes here.