A username is a name that identifies a user in a computer system or a website.
Usernames are often unique, meaning two users cannot share the same username. Also, usernames are usually paired with a password, and both may be required to sign in to a computer or website.
Usernames come in many forms. For example, on Twitter, usernames have to be unique, and start with an @ symbol. So @freeCodeCamp means that the username is freeCodeCamp.
But for an email address, your account's username is everything before the @ symbol. So for the email address firstname.lastname@example.org, the username is ghost.