marker

One that marks or serves as a mark, as.

A bookmark.

A tombstone.

A milestone.

An implement, especially a felt-tipped pen, used for marking or writing.

One who marks objects, especially for industrial purposes.

One who grades student papers.

A device, such as a line, stake, or flag, set on a playing field and showing the playing or scoring position.

A player who guards an opponent, as in soccer.