An unguided bomb, also known as a free-fall bomb, gravity bomb, dumb bomb, or iron bomb, is a conventional aircraft-delivered bomb that does not contain a guidance system and hence, simply follows a ballistic trajectory.
This described all aircraft bombs in general service until the latter half of World War II, and the vast majority until the late 1980s.
Then, with the dramatically increased use of precision-guided munitions, a retronym was needed to separate 'smart bombs' from free-fall bombs. 'Dumb bomb' was used for a time, but many military circles felt it sounded too trite, and eventually 'gravity bomb' gained currency. Previously, they were also referred to as 'iron bombs'.
Unguided bombs typically use a contact fuse for detonation upon impact. One alternative is a fuse with an altimeter, to cause an air burst at the desired altitude.
A special type of gravity bomb is the retarded bomb, which uses a mechanical method of creating increased drag, such as a parachute, ballute, or drag-inducing petals. These deploy after the ordnance is released, slowing its fall and abbreviating its forward trajectory, giving the aircraft time to get clear of the blast zone when bombing from low altitudes or with nuclear ordnance. However, these bombs are less accurate than conventional free-fall bombs.