Hotline means most recent, or “hottest responding” names. A 30 day hotline is names of new buyers/subscribers added to list in past 30 days. You’ll also see 60 day, 90 day and 6 month hotline. Some list owners also use hotline to describe past 12 months buyers, but that is not a true hotline. Other list owners call their hotlines a “recency select”.
Hotline names typically cost an additional $5/M to $15/M as they are the best responding and most valuable names on any list.