"Anonymous Coward" is a dummy name attributed to anonymous posts used by some weblogs that allow posting by people without registering for accounts. The practice originated on Slashdot, where the mildly derogatory term is meant to chide anonymous contributors into logging in. Some weblog engines such as Scoop use the term "Anonymous Hero" instead, perhaps to avoid the name's confrontational nature.
On systems where all users who are not logged in share one such account name, confusion may often result in discussions. It is also notable that in Slashdot's moderation system, "Anonymous Cowards" post at a default score of zero, which means that their posts are hidden from the default view unless they are moderated up by other users.
Both Slashdot and Scoop (when so configured) allow even logged in users to post anonymously, thereby providing selective anonymity (e.g. to avoid embarrassment).
Variations on the name "Anonymous Coward" are also sometimes used by trolls either to mock the dummy name or confuse other users into thinking that they are posting as Anonymous Coward.
