gnu privacy guard

Definition

also GnuPG; is a free-software replacement for Symantec's PGP cryptographic software suite. It is compliant with RFC 4880, the IETF standards-track specification of OpenPGP. Modern versions of PGP are interoperable with GnuPG and other OpenPGP-compliant systems.
More on wikipedia
See more about the closely related and often-confusing term PGP.