1. NSSL - Noun
2. NSSL - Proper noun
NSSL
(US, politics, military, espionage, aerospace) Initialism of National Security Space Launch program.; a U.S. government program for assured access to space
Coordinate term: EELV
NSSL (plural NSSLs)
(US, politics, military, espionage, aerospace) Initialism of national security spacelaunch.; a high security spacelaunch of U.S. government payloads, such as spy satellites
Then last year, it was revealed that SpaceX had sued again over the phase one NSSL contracts, which awarded money to help companies develop the rockets that they would use to apply for the phase two contracts to actually perform launches. Source: Internet