NSAabbr. National Security AgencyAmerican Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.NSA abbreviation for 1. (Government, Politics & Diplomacy) National Security Agency: a US federal defence bureau established in 1972 that specializes in cryptography 2. (Military) National Security Agency: a US federal defence bureau established in 1972 that specializes in cryptography Collins English Dictionary ? Complete and Unabridged, 12th Edition 2014 © HarperCollins Publishers 1991, 1994, 1998, 2000, 2003, 2006, 2007, 2009, 2011, 2014