In bittorrent, leeching is only downloading, while not sharing pieces of what you have downloaded with others. This is, however, luckily practically removed by the use of a tracker, which monitors your up/down ratio and speeds up your downloads if you upload more, or even doesn't let you download at all if you don't maintain a certain quota.
More generally, it is used as a term for certain types of bandwidth theft
In video games, leeching refers to the practice of a player joining a group for the explicit purpose of gaining rewards while negligibly contributing to the efforts necessary to acquire those rewards. This behavior may be accepted in efforts to powerlevel players. Usually it is considered poor behavior to leech without permission from the group.
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