A sniper was a type of marksman.
Snipers were among the Bannermen led by Gavrok. (TV: Delta and the Bannermen)
Missy recommended eight snipers, with two trained on each of her hearts and three for her brain stem, so that Clara Oswald could feel safe while speaking to her, noting that they'd have to "switch [her] off fast" before she could regenerate. (TV: The Magician's Apprentice)
One sniper affiliated with an intelligence organisation was stationed in Ivory Coast and was attacked by the Kasaavin while attempting to fire on a truck. (TV: Spyfall) Another killed "C" while he was briefing Team TARDIS about the Kasaavin attacks. (TV: Spyfall)