A sniper is a person trained to shoot a target from a long distance. They often are also skilled in the use of camouflage, stealth, concealment and reconnaissance.
The designated marksman is a related concept.
Snipers often use specialised equipment to fulfill their missions.
Starfleet's Military Assault Command Operations branch included snipers. In 2412 MACO Delta operators used TR-116B rifles in at least two assassinations in the Moab Confederacy. (The War of the Masters: Comes the Fall, A Change)
Lieutenant (JG) K'lak, son of Rokar on the crew of USS Bajor was trained as a sniper and favored a Type 4S phaser rifle. On away missions Lieutenant (JG) Kaitlyn McMillan would act as his spotter. (Bait and Switch: Bait and Switch)
During the Battle of Kobali Prime, Captain Kanril Eleya was lightly wounded in the shoulder by a female Vaadwaur sniper. She returned fire and killed her attacker. (Bait and Switch: "All of My Scars")