A spam trap is an email address that exists only to catch senders using poor list practices. Hitting one signals to mailbox providers and blocklists that you did not earn or clean your list, and it can blacklist your domain fast.
Two kinds
Pristine traps are addresses never used by a human, so mail to them proves scraping or guessing. Recycled traps are once-real addresses that were abandoned and repurposed, so hitting them proves you are not cleaning your list. Both are landmines in bought or scraped data.
How to avoid them
Never buy or scrape lists, verify addresses before sending, and remove long-inactive contacts. Spam traps are a direct route to a blocklist, which is why bounce hygiene and permission-based sending protect sender reputation.