Hitscan in video game design, most commonly in first-person shooters, is a type of hit registration system that determines whether an object has been hit or not simply by scanning if the item used was aimed directly at its target and then applies the effects of the item (usually damage) instantly.
Shooting the weapon calls the hitscan function, and if an object is detected in the projectile's path, a hit is registered.
In terms of game design, it readily ties cause (the player presses a 'fire' button, executing a hitscan function) to effect (the hitscan returns a result, the player sees the weapon's effect at that location).
As a heavily simplified model of real world ballistics, it makes games more accessible in that there is no need to aim slightly ahead of a moving target in order to compensate for the time it takes for the projectile to reach it.
With advances in processing and Internet bandwidth, it has become more practical to simulate the ballistic nature of real-world firearms in real-time games by using a more realistic "projectile" model, spawning bullets as actual game objects with mass and velocity, and continuously simulating them until they reach their target.