A spam trap is an address that exists only to catch senders who do not check their lists. Some were never used by a real person. Others belonged to real people, were abandoned, and later got reactivated as traps.

Lists that were purchased or scraped from old web pages are where traps tend to collect. A single hit can be enough for a blocklist to take notice.

uChecker's page on spam traps covers the three main types, how they get into lists and what reduces the risk.