mark·er 
(märkər)
(märkər)n.
1. One that marks or serves as a mark, as:
a. A bookmark.
b. A tombstone.
c. A milestone.
2. An implement, especially a felt-tipped pen, used for marking or writing.
3. One who marks objects, especially for industrial purposes.
4. One who grades student papers.
5. Sports
a. A device, such as a line, stake, or flag, set on a playing field and showing the playing or scoring position.
b. A player who guards an opponent, as in soccer.
c. An official in certain court games, such as squash, who mainly judges whether the ball is out of play and whether a fault or let has occurred.
6. Games
a. One that keeps score in various games.
b. A score in a game.
7. Slang A written, signed promissory note.
8. A chemical or physical characteristic that indicates the presence of something, especially:
a. A genetic marker.
b. A biomarker.
9. Linguistics An element that indicates grammatical or semantic class or function, such as an inflectional affix.