In BitTorrent sharing, a seed is a BitTorrent user who has 100% of a file and is sharing it for other BitTorrent users to download. A leech, on the other hand, is a BitTorrent user who downloads the files shared by the seeds and does not seed back to other users.
The download time for a file shared via BitTorrent depends on the number of seeders available for that file; more seeders mean higher torrent speed.
Techopedia explains Seed
In addition to seeders and leechers, there are peers who are users downloading a file, have part of it, yet are simultaneously uploading the downloaded part to other users. Even though the peers share back the files simultaneously, the actual torrent speed mainly depends on the number of seeders available for the shared file.
When downloading a file, a leech or a peer is not downloading the file from the actual site it is in, but is downloading it from a seed’s computer.
For example, assume that a seed is seeding a file at 50 kbps, and there are five leechers to download that file. Initially, the file is shared at 10 kbps each for every leech. When the leechers finish downloading, and if they seed back the file at the same speed of 50 kbps, the total torrent speed increases to 300 kbps. This process continues depending on the number of seeders and leechers for that specific file.
On the contrary, if the leechers decide not to seed the file after completely downloading it, the original seeder is stuck as the only one seeding the file. The more seeds, the better the download rate. However, it is good to have more peers in addition to the seeders, as the downloaders can utilize both.
Once a file is fully seeded, the BitTorrent application automatically stops the seeding process and the file can then be removed from the seeding list.
This definition was written in the context of BitTorrent