An object the sailing instructions require a boat to leave on a specified side, and a race committee boat surrounded by navigable water from which the starting or finishing line extends. An anchor line or an object attached temporarily or accidentally to a mark is not part of it.
Room for a boat to sail to the mark, and then room to sail her proper course while at the mark. However, mark-room does not include room to tack unless the boat is overlapped to windward and on the inside of the boat required to give mark-room.
The video below brings clarity to what is meant by that tiny little word "at" in the above definition. When a boat must start to change course in order to round the mark on her proper course then she is "at" the mark. Unfortunately this is not a printed definition and so we might expect to see a call book addition at some point in time to make this abundantly clear.