Hitscan

Hitscan is an algorithm use to determine what is "hit" in a straight trajectory given the angle, and is used in Doom for some attacks. A hitscan attack hit or miss its target the very moment the it is launched, and therefore cannot be dodged. There are no projectiles involved, invisible or otherwise.

All bullet attacks work like this, in addition to the player fist and chainsaw melee weapons. Furthermore the subsequent tracer rays from the BFG are also hitscan attacks. Up to Doom 1.4, demons also used hitscan attacks, however this was changed to a range check in v1.5.

Bullet and melee attacks have ranges of 2048 and 64 units respectively. The BFG tracers have a range of 1024 units. Excluding the randomization, hitscan attacks have a fixed damage and doesn't vary depending of the distance.

In addition, the damage dealt by player and monster bullet hitscan attacks is slightly different. While the maximum damage is identical, the minimum damage is a bit lower for monsters.

Range check
Monster melee attacks use a simple range check, whereas bullet attacks and player melee attacks use a hitscan line computation system to make sure there is no unwanted target in-between the attacker and its victim. Contrarily to hitscan attacks, a range check has no risk of hitting an unintended target.

Demons used a short-range hitscan attack in Doom 1.4, allowing for the occasional (but rare) demon-started infighting. This was changed to a range check in v1.5, consistent with the imp and baron of hell melee attacks.

The lost soul in the beta version of Doom used a long-range melee attack consisting merely of a sight check.