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. Sports A device, such as a line, stake, or flag, set on a playing field and showing the playing or scoring position. Sports A player who guards an opponent, as in soccer. Sports 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. Games One that keeps score in various games. Games A score in a game. Slang A written, signed promissory note. A genetic marker. Medicine A physiological substance, such as human chorionic gonadotropin or alpha-fetoprotein, that when present in abnormal amounts in the serum may indicate the presence of disease, as that caused by a malignancy. Also called biomarker. Linguistics An element that indicates grammatical class or function; a derivational or inflectional morpheme.