The BFG 9000, short for Blast Field Generator 9000 (Bio Force Gun version 3.14 in the 2005 movie), or more commonly known as the Big Fucking Gun 9000, is a weapon featured consistently throughout the Doom games.
It is Doomguy's most powerful weapon, firing a large green energy ball that obliterates anything it touches and emits tracers for extra damage.
In the original Doom series, the BFG works in an unintuitive way. When the trigger is pulled, there is a slight delay before a large green plasma ball is ejected. After the green ball hits any object, there is another slight delay before 40 invisible tracers are emitted from the player's body in a cone shape, like a shotgun blast, in the same direction the plasma ball was originally fired. The green ball itself deals significant damage to enemies when hit, but most of the potential damage from a BFG shot is from the tracers. Sufficiently skilled players can take advantage of this and consistently kill a Cyberdemon, a boss monster with the most health in the game, in two BFG shots. This requires getting very close at the right times to maximize the number of tracers hitting the enemy, which is risky because the cyberdemon's rocket launcher is extremely damaging, guaranteed to one-shot players who have no armor or extra health.
