NIST SP800-38D.pdf

This Recommendation specifies the Galois/Counter Mode (GCM), an algorithm for authenticated encryption with associated data, and its specialization, GMAC, for generating a message authentication code (MAC) on data that is not encrypted. GCM and GMAC are modes of operation for an underlying approved symmetric key block cipher.
KEY WORDS: authenticated encryption; authentication; block cipher; confidentiality, cryptography; encryption; information security; mode of operation.