Sniping

Sniping is the act of using a rifle to hit a target at long range. Most sniping is done with the help of a telescopic sight to accurately see your target and make adjustments to compensate for distance and wind. The word sniper comes from the hunters who hunted birds of the similar name called snipe.  These birds were very hard to hunt and shoot.  

A marksmen talented enough to kill a snipe was then dubbed  a "sniper" by others. 
Sniping is a legitimate tactic used in many first and third person shooters. It can generally be associated with camping as sniping well requires a good vantage point of the map and usually requires to work at an enemy's flank or protect an object from a vantage point. 
Staying hidden from enemies is also important with sniping. 
Examples of hiding would be to take cover in foliage or a structure and staying still or moving slowly to change position as to not let the enemy notice suspicious movement. Sniping also requires patience and is the exact opposite of "running-and-gunning" in terms of play style. 
A recent example of sniping would be in Battlefield:Bad Company, in which the sniper class comes with a ghille suit which provides crucial cover in foliage. The huge maps also allow for some effective flanking on enemy positions. Also moving while looking in the scope will usually cause the bullet to fire off the crosshairs position, so staying still preserves most of the accuracy.
 
Gameplay Variations 
 Many early games, such as Rescue: The Embassy Mission for the Nintendo Entertainment System featured specific segments dedicated to sniping.  These segments allow you to do nothing but snipe with reduced risk of injury to your avatar.
 Other games that employ this method are  Mission Impossible for the Nintendo 64, where you protect a non-player character by identifying and neutralizing threats, and Wanted: Weapons of Fate, where player movement is restricted when a sniper rifle is encountered and the player must priororitize targets to survive.
 
MDK allowed the player to zoom in through the character's helmet without selecting a specific weapon.
 
However, most games feature a sniper rifle as a specific weapon.