In computing and specifically in Internet slang, a leech is one who benefits, usually deliberately, from others' information or effort but does not offer anything in return, or makes only token offerings in an attempt to avoid being called a leech. In economics, this type of behavior is called "free riding" and is associated with the free rider problem. The term originated in the bulletin board system era, when it referred to users that would download files and upload nothing in return.
Depending on context, leeching does not necessarily refer to illegal use of computer resources, but often instead to greedy use according to etiquette: to wit, using too much of what is freely given without contributing a reasonable amount back to the community that provides it. The word is also used without any pejorative connotations, simply meaning to download large sets of information: for example the offline reader Leech, the Usenet newsreader NewsLeecher, the audio recording software SoundLeech, or LeechPOP, a utility to download attachments from POP3 mailboxes.
The name derives from the leech, an animal that sucks blood and then tries to leave unnoticed. Other terms are used, such as "freeloader", "mooch" and "sponge", but leech is the most commonly used.
Wi-Fi leeches attach to open wireless networks without the owner's knowledge in order to access the Internet. One example of this is someone who connects to a café's free wireless service from their car in the parking lot in order to download large amounts of data. Piggybacking is a term used to describe this phenomenon.
Direct linking (or hot-linking) is a form of bandwidth leeching that occurs when placing an unauthorized linked object, often an image, from one site in a web page belonging to a second site (the leech).
In most -networks, leeching can be defined as behavior consisting of downloading more data, over time, than the individual is uploading to other clients, thus draining speed from the network. The term is used in a similar way for shared FTP directories.