S/MIME in Internet technology stands for

Concept: MIME (Multipurpose Internet Mail Extensions) is the internet-mail framework that lets a message body carry arbitrary content types — many character sets, images, audio, video and attachments — through headers such as Content-Type and Content-Transfer-Encoding. A security extension layered on top of MIME adds cryptographic services to those body parts: digital signatures for authentication, integrity and non-repudiation, and encryption for confidentiality.

Application: S/MIME is exactly that security layer over MIME. The leading S supplies the security qualifier, and the remainder of the acronym is MIME's own expansion, so the full form reads Secure/Multipurpose Internet Mail Extension(s). In operation it takes a MIME body part, applies CMS / PKCS #7 cryptographic processing, and re-wraps the result in MIME media types such as application/pkcs7-mime and application/pkcs7-signature, using X.509 certificates issued by a certification authority.

Cross-check: why the other two words do not belong in this acronym:

• Simple belongs to SMTP — Simple Mail Transfer Protocol, the protocol that transfers the message between servers — not to MIME.

• Multimedia names a class of content (audio, video, images), whereas MIME is defined as Multipurpose because it covers arbitrary content types, media or not.

• PGP / OpenPGP provides comparable mail signing and encryption, but through a web of trust rather than X.509 certification authorities.

Result: S/MIME is the security-qualified form of MIME, so its expansion is Secure Multipurpose Internet Mail Extension in the wording used by this paper. The standards-canonical spelling — RFC 5751 and its successor RFC 8551 — is Secure/Multipurpose Internet Mail Extensions, with a slash after Secure and a plural noun; that difference is purely presentational, the same singular ending appears in every wording offered here, and the substantive point is that S stands for Secure while MIME stands for Multipurpose Internet Mail Extensions.