What does the SSN abbreviation mean in the Navy?

All Navy vessels and aircraft have designations on them indicating their purpose, hull type (when applicable) and propulsion type, and in most cases, tactical use.

"SSN" is the hull abbreviation for a Nuclear Powered Fast-Attack Submarine. The letters read out as:

S - Submersible

S - Ship

N - Nuclear

Or, Submersible Ship, Nuclear.

For tactical examples, an SSBN is a Ballistic Missile Submarine; SSGN is a Cruise (Guided) Missile submarine.