Distributed hash table - ButTorrent extension for discovering torrent's seeders by advertising its hash across known peer pool, think of it as a distributed tracker. Contrary to traditional way of asking a known tracker for peers of that torrent.

Its algorithm is very elegant, using binary search on peers' and torrents' hashes, narrowing down to peers that are more likely to be seeders (or at least know some).

https://www.bittorrent.org/beps/bep_0005.html