An intercontinental ballistic missile (ICBM) is a ballistic missile with a minimum range of more than 5,500 kilometres (3,400 mi) primarily designed for nuclear weapons delivery (delivering one or more nuclear warheads). Similarly conventional, chemical and biological weapons can also be delivered with varying effectiveness. Most modern designs support multiple independently targetable reentry vehicles (MIRVs), allowing a single missile to carry several warheads, each of which can strike a different target.
Early ICBMs had limited accuracy that allowed them to be used only against the largest targets such as cities. They were seen as a 'safe' basing option, one that would keep the deterrent force close to home where it would be difficult to attack. Attacks against military targets, if desired, still demanded the use of a manned bomber. Second and third generation designs dramatically improved accuracy to the point where even the smallest point targets can be successfully attacked.
ICBMs are differentiated by having greater range and speed than other ballistic missiles: intermediate-range ballistic missiles (IRBMs), medium-range ballistic missiles (MRBMs), short-range ballistic missiles (SRBMs)—these shorter range ballistic missiles are known collectively as theatre ballistic missiles. There is no single, standardized definition of what ranges would be categorized as intercontinental, intermediate, medium, or short.