a remarkably skilled marksman in real life, or in a video game
1)in the video game scenario, he/she is usually considered "cheap", a "noob", or a coward
(s)he usually pisses off those of the other team by sniping them from halfway across the map from a very covert location.
although (s)he is a threat from far away, one can strafe back and forth to avoid being shot while closing in on his/her location, and if the player gets to close enough to the sniper, the sniper is rendered virtually useless because "unscoped" shots are particularly hard to pull off
2)a sniper may also piss off his teammates, too, by shooting the person the player is sneaking up on, therefore making the sniper a "kill stealer"
*player suddenly drops dead, no visible killer in sight*
Player: god**** snipers!