v.1. To indicate the boundary of some region: The rows of trees on either side of the property mark off our part of it. The lime marks the playing field off.
2. To make a mark or sign by the name of someone or something on some list, especially to indicate completion of a task or an intention to do a task: As I delivered each package, I marked off the name of the addressee to make sure I hadn't forgotten anyone. Well, that task is finished, so let's mark it off the list.
3. To lower someone's score or grade by some amount due to some error: My professor marked me off a grade for being late. The teacher marks off five points for each spelling error. The judges marked the dancers off for dancing over the time limit.