There is no general category for "defensive nuclear weapons"although they were once envisioned as part of an air-defenseagainst massed bombers. The 3 letter acronym for a "defense againstnuclear missiles" is ABM, which stands for Anti-Ballistic Missiledefense.(There is a related acronym from the Cold War, MAD, for MutualAssured Destruction, a policy to preclude a first strike in nuclearwar by assuring the survival of retaliatory weapons.)