It means that the user name which you have chosen is void on the grounds of it already existing or due to the use of certain symbols which are prohibited.
Identification
user name is screen name
Every user account has its own directory. Every user on a system has a unique account name. You must have a valid user account to start a UNIX session.
If it's a password lock out there is nothing you can do. The installation disk will allow you to reset the password.
It means that the program you are tying to log onto does not recognise the User Name you have entered. Enter the correct User name.
it means that he/she is a facebook user ??
A user name in this site looking at questions when she is here. A user name in this site looking at questions when she/he is here.
Identification
If a username is blue, the user is on your friend list.
That you typed in the wrong user name that you're looking for, and/or that it doesn't exist.
No and yes, no it will come up with who answered it's user name but your user name doesn't necessarily mean that it's your real name.
To insure that the users input is valid.