UserID

A UserID or UID is the unique number given to your LiveJournal account when it is created. Your UserID is to the right of your username in the User: section of your User Info. UserIDs are created sequentially as more accounts are created, so lower UserIDs are older accounts, and higher UserIDs will be newer accounts.

Note that you cannot find out what LiveJournal username corresponds to a given UserID--you can only find the UserID for a given username. This was to prevent abuse of the system. (Imagine spammers cycling through every single journal profile to gather emails, for instance.) If you are doing research and need special permissions, you should contact LJ as defined on the bot policy page.