Enemy Tagging is a feature seen in some multiplayer games that allows players to intentionally point out enemy locations for other teammates with an onscreen indicator. In some games, only certain classes have this ability, but often its something everyone can do.
When an enemy is spotted, an indicator appears above the enemy's head for all teammates to see, and sometimes will stay there even if the enemy goes out of sight. Bonus points are often awarded to the spotter if a fellow teammate kills the spotted enemy.
It is also sometimes seen in single player games, where a player can point out the location of enemies for computer-controlled teammates to attack, or simply to track an enemy for one's own benefit.
*This concept does not include games where enemy locations show up automatically (for example, on a mini-map when their gun is fired).