Secure Multipurpose Internet Mail Extensions definition in Computer Security terms:
Definition(s): A protocol defined in IETF RFCs 3850 through 3852 and 2634 for encrypting messages and creating certificates using public key cryptography. S/MIME is supported by default installations of many popular mail clients. S/MIME uses a classic, hierarchical design based on certificate authorities for its key management, making it suitable for medium to large implementations.
Source(s): NIST SP 800-45 Version 2
reference: CSRC Glossary