A no-scope is a frag with a sniper rifle that is not fully scoped in. Simply put, in first-person shooters it is a shot with a sniper from the hip, i.e. without using the crosshairs, which hits and eliminates the opponent.
"Scope" is the term used to describe the telescopic sight on sniper rifles. The term "No Scope" is therefore derived from the fact that one does not use the scope for this shot.
The no-scope phenomenon has been around as long as first-person shooters have existed. By many players, a no-scope is considered the ultimate proof of skill. The probability of hitting a target decreases with increasing distance, the long reload time of snipers usually makes you pay for a miss with your own life, and the necessary confidence in your own abilities is often based on a fundamentally higher skill level.
A special variant is the 360 no-scope, where the player turns 360 degrees before firing. This form of shooting is considered the absolute supreme discipline by many shooter fans. Often a 360 no-scope is combined with a jump from top to bottom. The 360 was made big by Call Of Duty 4 player zzirGrizz, among others, who included many 360 no-scope frags in his gameplay videos. However, over the years there were of course more and more players who dedicated themselves to the 360 No-Scope, so that you see it more often today.
What is possible depends heavily on the game and weapon mechanics. Shooters like Call Of Duty often make it easier, because they don't put so much emphasis on realism in certain movement sequences and the gameplay is more important.