username definition by American Heritage Dictionary
username - Computer Definition
Synonymous with user ID. See user ID.
The name people use to identify themselves when logging into a computer system or online service. Both a username (user ID) and a password are required. In an Internet e-mail address, the username is the left part before the @ sign. For example, KARENB is the username in email@example.com. AOL calls a username a "screen name."