UCE stands for "Unsolicited Commercial Email", more commonly known as spam.
CAUCE (Coalition Against UCE) has a segment in their FAQ defining spam as:
The definition of "spam" is a tricky issue, with as many strongly held opinions as many other age old questions such as "the number of angels who can dance on the head of a pin" and "chicken versus egg." For example, many define spam as unsolicited electronic mail sent in bulk. Others believe "bulkness" is irrelevant, it's merely a matter of whether the message sent was solicited. Still others debate the importance of whether the message was commercial in nature.
