Unsolicited means that the Recipient has not granted verifiable permission for the message to be sent. Bulk means that the message is sent as part of a larger collection of messages, all having substantially identical content.

Unsolicited Email is normal email (examples: first contact enquiries, job enquiries, sales enquiries)

Bulk Email is normal email (examples: subscriber newsletters, customer communications, discussion lists)

The recipient's personal identity and context are irrelevant because the message is equally applicable to many other potential recipients; AND

The recipient has not verifiably granted deliberate, explicit, and still-revocable permission for it to be sent.

Spam is an issue about consent, not content. Whether the UBE message is an advertisement, a scam, porn, a begging letter or an offer of a free lunch, the content is irrelevant - if the message was sent unsolicited and in bulk then the message is spam.

Spam is not a sub-set of UBE, it is not "UBE that is also a scam or that doesn't contain an unsubscribe link", all email sent unsolicited and in bulk is Spam.

This distinction is important because legislators spend inordinate amounts of time attempting to regulate the content of spam messages, and in doing so come up against free speech issues, without realizing that the spam issue is solely about the delivery method.

Various jurisdictions have implemented legislation to control what they call "spam". One particular example is US S.877 (CANSPAM 2004). Each law addresses "spam" in different ways, and as a consequence, often has different definitions of what they cover, whether they call it "spam" or not. Spamhaus uses the industry standard "unsolicited bulk email" definition which underlines "it's not about content, it's about consent". As such, arguments as to whether UBE messages are covered under CANSPAM or are compliant with CANSPAM, are entirely irrelevant.