The traditional definition of a sniper is an infantry soldier especially skilled in field craft and marksmanship who kills selected enemies from concealment with a rifle at large distances. Typically and ideally, a proficient sniper approaches an unaware enemy presence, uses a single bullet per target, and withdraws without being seen. The word originates from the snipe, a game bird difficult for hunters to sneak up on.

In the last few decades the term 'sniper' has been used rather loosely, especially by the media in association with police precision riflemen, those responsible for assassination, any shooting from all but the shortest range in war and any criminal equipped with a rifle in a civil context. This has rather expanded the general understanding of the meaning of the term. It has also given the term 'sniper' distinctly pejorative connotations. This explains the increasing use of alternative terms, especially for police snipers such as counter-sniper, precision marksman, tactical marksman, sharpshooter and precision shooter.

Snipers in warfare

Different countries have different military doctrines regarding snipers in Military units, settings, and tactics. Generally, a sniper's goal in warfare is to reduce the enemy's ability to fight by carefully striking at very few, high value targets.

Soviet, Russian, and derived military doctrines include squad-level "snipers," which may be called "sharpshooters" or "designated riflemen" in other doctrines (see below). They do so because this ability was lost to ordinary troops when assault rifles (which are optimized for close-in, rapid-fire combat) were adopted. See the "Soviet sniper" article for details.

Military snipers from the U.S., U.K. and derived doctrines are typically deployed in two-man teams consisting of a shooter and spotter. The role of the sniper typically goes to the more experienced soldier, but the opposite is also a valid tactic, as a more experienced spotter will calculate range and wind with more accuracy.

Typical sniper missions include reconnaissance or scouting and surveillance, anti-sniper, killing enemy commanders, selecting targets of opportunity, and even anti-matériel tasks (destruction of military equipment), which tend to require use of rifles in the larger calibres such as .50 BMG and .338 Lapua. Snipers have of late been increasingly demonstrated as useful by U.S. and U.K. forces in the recent Iraq campaign in a fire support role to cover the movement of infantry, especially in urban areas.

The current record for longest range sniper kill is 2,430 metres (7,972 ft), reportedly accomplished by a Canadian sniper in 2002, during the invasion of Afghanistan, using a .50 BMG McMillan bolt-action rifle. This meant that the round had a flight time of four seconds, and a drop of 44.5 m (146 ft). The previous record was held by Carlos Hathcock, achieved during the Vietnam War, at a distance of 2,250 m.

Such a shot cannot be taken in haste. By contrast, much of the U.S./Coalition urban sniping in support of operations in Iraq is at much shorter ranges, although, in one notable incident on April 3, 2003, a two man team of Royal Marines armed with L96 sniper rifles each killed targets at a range of about 860 m with shots which dropped 17 m (56 ft) in the air.

In the Bosnian War, and for much of the Siege of Beirut, the term "sniper" was used to refer to what were generally ill-trained soldiers who terrorized civilians. During the Siege of Sarajevo, the main street of the city became known as "Sniper Alley".

Generally snipers are isolated even from soldiers of their own army by the dislike of the ordinary infantry for this type of combat. During World War II, captured snipers were often shot out-of-hand by their captors.