Spam
Unsolicited bulk email: mail sent to many recipients who did not give verifiable permission, whatever its content. Laws and providers each draw the line differently.
What it means
Spamhaus defines spam as a message that is both unsolicited and bulk: the recipient's identity is irrelevant because the same message applies to many others, and the recipient has not verifiably granted deliberate, explicit and still-revocable permission. On that view spam is about consent, not content, and a lawful advertisement can still be spam.
Other definitions are narrower or wider. For feedback loops, RFC 6449 calls spam any message a recipient complains about. Laws such as CAN-SPAM regulate commercial email without requiring prior consent, while EU law requires consent for marketing email to individuals, with exceptions.
For deliverability the working definition is the provider's: mail its users report or ignore gets filtered, whether or not it was legal to send.
Related terms
Sources
What this definition rests on. The wording is ours; the facts are theirs.
All 432 terms in the glossaryThe wiki: blocklists and bounce codes