From Halopedia, the Halo wiki
Snapshot is a medal awarded in Halo 4 and Halo 5: Guardians multiplayer and Spartan Ops for killing an enemy with a Sniper Rifle, Beam Rifle or Binary Rifle while not zoomed in, a tactic commonly known as "no-scoping". It is represented by a Sniper Kill medal with a red "X" in the center of the crosshairs.
If an AI passenger armed with a sniper weapon kills an enemy, the player driving will be awarded a medal for that sniper kill, and because the player is not zoomed in they will also earn the Snapshot medal. Similarly, if the player shoots an enemy with the sniper rifle, and then zooms out (or is even killed) and the enemy is then finished off by an AI ally, fall damage, or friendly fire, the player also will be awarded the Snapshot medal.