Retention

Retention is the measure of how long a torrent remains seeded for. If a tracker has long retention times then the torrents last a long time before running out of seeds, usually many years for even unpopular torrents. Adversely, short retention times for a tracker means that torrents don't remain seeded for long, which is bad.

Private trackers generally have much better retention than public trackers due to incentives and rules that make users seed for longer periods of time.