Pseudonym
From Telecomix Crypto Munitions Bureau
A pseudonym is a name that only a selected group of others know who is behind it (zero or more people/robots). It can be thought of a placeholder for your real name. Examples: Nicknames and pen names.
[edit] Post-intertubes meaning of the word
Alternatively, a pseudonym is any name that one uses. It becomes pointless to say that it is a placeholder for your "real" name if you only have pseudonyms.
Example: The government thinks you are named Forename Middle_name Lastname followed by an optional serial number (depending on country), and everyone else thinks you are named various Yournicknames. In this case, its obviously not correct to stick to the pre-intertubes definition of the word "pseudonym".
[edit] Random numbers and pseudonymity
TOR and I2P are not anonymous, they are pseudonymous. Identities consists of public cipher keys optionally modulated through a hash algorithm (That is, the identity one wears is a seamingly random number). Verification of identity is guaranteed by the possession and usage of the private key: Only the user/robot with the corresponding private key can reply to a message sent to the public key.
Rapid generation of new identities, for example if a new and unique number-pseudonym is used for each new connection, essentially makes this form of pseudonymity equal to anonymity.
