Username or User Name?

This comes across a million time when I am putting a placeholder in a login page. What should I use, Username or User Name?

Well Both are correct but there is a small difference when we see from users perspective.

“Username” is the unique word that a user created while registering and that he will type in with his password, for example: bobsmith. And mostly website use this.

But “User Name” is the name of the user, the user’s real life name, for example: Bob Smith.