Nuclear weapons, or nukes for short, are the most horrific weapons in the game. There are two types of nukes: ICBMs (inter-continental ballistic missiles) and Tactical Nukes, the latter of which were introduced in Beyond the Sword.
Using a nuke will destroy or damage units in the tile the nuke hits and in the eight adjacent tiles. As a result of detonating a nuclear weapon there will also be radioactive fallout in the targeted area. The AI players will also get a negative attitude modifier against you if you use nukes.