Pretty Good Privacy
a program used for digitally signing and encrypting data, but especially email.
This is part of a Public Key Infrastructure (see PKI), using PublicKeyEncryption.
There is an inter-operable open-source (ok, free software...) version called GPG.
For notes on how to use these programs, see GPG/PGPNotes
PGPCorporation, a commercial company that sells PGP software and services and runs the PGPGlobalDirectory, sometimes trades a PGP.
Part of CategorySecurity