From Virtual Entity of Relevant Acronyms (Version 1.9, June 2002) :
UCE
Unsolicited Commercial E-mail (Usenet, spam, UBE)
From Jargon File (4.3.1, 29 Jun 2001) :
UCE n. [abbrev., Unsolicited Commercial Email] A widespread, more
formal term for email spam. Compare UBE, which may be superseding
it.