This is a glossary of jargon related to peer-to-peer file sharing via the BitTorrent protocol.
Terms
Availability
(Also known as distributed copies.) The availability of each piece in the torrent is defined as the number of peers who have a copy of that piece. The availability of the entire torrent is defined as the nonnegative real number whose integer part is the minimum piece availability and whose fractional part is the fraction of pieces that have higher availability than the minimum piece availability. Example: There are 10 pieces, Peer A has pieces 0 to 5, Peer B has 2 to 7, and Peer C has 4 to 9. Pieces 0, 1, 8, 9 have availability 1. Pieces 2, 3, 6, 7 have availability 2. Pieces 4 and 5 have availability 3. The entire torrent has availability 1.6 (1 + 6/10). The integer part is 1 because 1 is the lowest piece availability. The fractional part is 6/10 because more than one peer has pieces 2 to 7 (6 pieces) and there are 10 total pieces. Even though 3 peers have pieces 4 and 5, it does not further increase the availability. Sometimes "distributed copies" is considered to be "availability minus 1". So if the availability is 2.6, the distributed copies will be 1.6 because it is only counting the additional "copies" of the file.
Choked
Describes a peer to which the client refuses to send file pieces. A client chokes another client in several situations: The second client is a seed, in which case it does not want any pieces (i.e., it is completely uninterested) The client is already uploading at its full capacity (it has reached the value of max_uploads) The second client has been blacklisted for being abusive or is using a blacklisted BitTorrent client.
Client
The program that enables peer-to-peer file sharing via the BitTorrent protocol. See Comparison of BitTorrent clients.
Distributed Hash Table
Distributed Hash Tables (DHT) are used in Bittorrent for peers to send a list of other seeds/peers in the swarm for a particular torrent directly to a client without the need for a tracker.
Downloader
A downloader is any peer that does not have the entire file and is downloading the file. This term, used in Bram Cohen's Python implementation, lacks the negative connotation attributed to leech. Bram prefers downloader to leech because BitTorrent's tit-for-tat ensures downloaders also upload and thus do not fairly qualify as leeches.
Endgame / Endgame mode
Any applied algorithm for downloading the last few pieces (see below) of a torrent. In typical client operation the last download pieces arrive more slowly than the others. This is because the faster and more easily accessible pieces should have already been obtained. In order to prevent the last pieces becoming unobtainable, BitTorrent clients attempt to get the last missing pieces from all of its peers. Upon receiving the last pieces a cancel request command is sent to other peers.
Fake
A fake torrent is a torrent that does not contain what is specified in its name or description (e.g. a torrent is said to contain a video, but it contains only a snapshot of a moment in the video, or in some cases malware).
Freeleech
Freeleech means that the download size of the torrent does not count towards your overall ratio, only the uploaded amount on the torrent counts toward your ratio.
Grab
A torrent is grabbed when its metadata files have been downloaded.
Hash
The hash is a digital fingerprint in the form of a string of alphanumeric characters (typically hexadecimal) in the .torrent file that the client uses to verify the data that is being transferred. "Hash" is the shorter form of the word "hashsum". Torrent files contain information like the file list, sizes, pieces, etc. Every piece received is first checked against the hash. If it fails verification, the data is discarded and requested again. Hash checks greatly reduce the chance that invalid data is incorrectly identified as valid by the BitTorrent client, but it is still possible for invalid data to have the same hash value as the valid data and be treated as such. This is known as a hash collision. Torrent and p2p files typically use 160 bit hashes that are reasonably free from hash collision problems, so the probability of bad data being received and passed on is extraordinarily small.
Health
Health is shown in a bar or in % usually next to the torrent's name and size, on the site where the .torrent file is hosted. It shows if all pieces of the torrent are available to download (i.e. 50% means that only half of the torrent is available). Health does not indicate whether the torrent is free of viruses.