Privacy-Enhanced Mail (PEM) is a de facto file format for storing and sending cryptographic keys, certificates, and other data, based on a set of 1993 IETF standards defining "privacy-enhanced mail."
While the original standards were never broadly adopted and were supplanted by PGP and S/MIME, the textual encoding they defined became very popular.
The PEM format was eventually formalized by the IETF in RFC 7468.