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An unguided bomb, also known as a free-fall bomb, gravity bomb, dumb bomb, or iron bomb, is an aircraft-dropped bomb (conventional or nuclear) that does not contain a guidance system and hence simply follows a ballistic trajectory. It includes all aircraft bombs in general service until the latter half of World War II, and the vast majority until the late 1980s, which were known simply as "bombs".
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 trite, and eventually "gravity bomb" gained popularity.
Bomb casings for unguided bombs are typically aerodynamic in shape, often with fins at the tail section, which reduce drag and increase stability after release, both of which serve to improve accuracy and consistency of trajectory.
Unguided bombs typically use a contact fuze for detonation upon impact, or some milliseconds after if a penetration effect is required. Alternatives include a fuze with an altimeter to cause an air burst at the desired altitude, and a proximity fuze to cause an air burst at a specific distance from the ground or other target.