Retention (Usenet)
How long a news server keeps articles before purging them — a couple of weeks in the 1980s, more than a decade for today's commercial providers.
Usenet relied on every server storing its own copy of everything, and disk was the scarce resource. A nightly expire run deleted articles past their age limit; in INN, expire.ctl set how long each hierarchy could stay, so comp.* might be kept longer than the junk. Go on holiday for two weeks and you could come back to a group with no beginning.
The server also remembered Message-IDs in a history file for longer than it kept the articles themselves — otherwise a late copy from a neighbour would be accepted all over again as new. Once the binaries groups took off, retention became a selling point, and providers that live on downloads now advertise theirs in thousands of days.