SLBM is the standard abbreviation for a submarine-launched ballistic missile.
An SLBM is a ballistic missile designed to be fired from a submarine, usually a nuclear-powered ballistic-missile submarine. These missiles form one part of a nation’s nuclear triad, alongside land-based missiles and aircraft.
The submarine platform gives SLBMs a major strategic advantage: a submerged boat can remain difficult to detect while patrolling far from its home port. This survivability supports the idea of a second-strike capability, meaning a country could respond even after suffering an initial nuclear attack.
The first successful launch of a ballistic missile from a submerged submarine was conducted by the United States in 1960 using Polaris. SLBM should not be confused with ICBM, which is launched from land, or cruise missiles, which generally fly along a lower, guided path.