Intercontinental ballistic missile

ICBM is the abbreviation for "InterContinental Ballistic Missile".

Background
An ICBM is a ballistic missile with a long range typically designed for nuclear weapons delivery (delivering one or more nuclear warheads). The missile launches up into low orbit, where it can then deploy one or more nuclear warheads on almost any location on the planet. They could be fitted with a variety of payloads, ranging from high explosive to biowarfare loadouts, but most common where nuclear warheads. A typical ICBM has a blast radius of several miles, and a fallout range of even further.

Fallout 3
ICBM silos can be found throughout the Capital Wasteland. A (non-functional) ICBM launch terminal, launch codes, and facility can be found at Fort Constantine. The launch codes are for a Minuteman XI missile.

Fallout: New Vegas
Raul Tejada recalls seeing Mr. House's anti-missile defense systems shoot down Chinese ICBM's during his unmarked quest Old School Ghoul.

ICBMs can also be seen throughout the Divide, stationary and while in flight before detonating, and their warheads can also be seen and detonated.