The word "username" refers to the combination of characters that identifies an authorized person and allows him or her access to a computer, an account or a network. A common example of a username is the identification characters that come before the "@" in an e-mail address.
Other names for username include sign-in name, sign-on name, user ID, user name (using two separate words rather than one), log-in name and log-on name. Sometimes a username is generated by the account allowing the access, but often it is the name chosen by the user. The letters, numbers, symbols or combinations give the person using the computer or account a unique identity.