Pseudo-anonymity is the appearance – but not the reality--of anonymity online.
Most commonly, pseudo-anonymity enables anonymous posting and commenting. Pseudo-anonymous users have no visible identifier, and information that can be linked to them is only available to service providers or site administrators (unless the users themselves provide such information in their messages). However, that doesn't mean that messages can't be traced back to their sources. Users are generally required to provide some form of ID for sign up and IP addresses are typically logged and saved.
Many sites or services that allow anonymous posting stipulate that the administrators can provide police with user IP addresses, GPS coordinates, device details and data and time for each message if supported by a search warrant, court order or subpoena. Some sites also make user info available to other businesses and advertisers.
While pseudo-anonymity can be good for civil liberties, privacy and security, some users may exploit it to be abusive to other users or to vandalize sites. On truly anonymous sites, such behavior can be hard to regulate as users can return almost indefinitely. On pseudo-anonymous sites, however, administrators have access to information that can be used to ban users and keep them from returning.
Pseudoanonymity is related to pseudonymity, the more commonly seen system, in which users are identified by something other than their actual names.
See also: Pseudonymous profile