Here's something else I stumbled across again today on Spamhaus, their definition of "Closed loop opt-in," which I like:

With Closed-Loop Opt-in the Recipient has verifiably confirmed permission for the address to be included on the specific mailing list, by confirming (responding to) the list subscription request verification. This is the standard practice for all Internet mailing lists, it ensures users are properly subscribed from a working address and with the address owner's consent.