The Collaborative International Dictionary of English v.0.48:

ICBM \ICBM\ n. [acronym intercontinental ballistic missile]
   An intercontinental ballistic missile, a long-range ballistic
   missile that is capable of traveling from one continent to
   another. Contrasted with IRBM.

   Syn: intercontinental ballistic missile.
        [WordNet 1.5]

The Collaborative International Dictionary of English v.0.48:

ballistic missile \bal*lis"tic mis`sile\, n.
   A rocket-propelled missile of long range which is guided only
   during the powered portion of its flight, which usually takes
   only a small part of the total flight time; -- contrasted
   with guided missile.

   Note: Ballistic missiles are sometimes referred to by their
         range, such as an

   intercontinental ballistic missile (ICBM) or an

   intermediate range ballistic missile (IRBM).
      [PJC]

WordNet (r) 3.0 (2006):

ICBM
    n 1: a ballistic missile that is capable of traveling from one
         continent to another [syn: intercontinental ballistic
         missile, ICBM]