This is a simple issue, but on every project the debate always comes up about the proper usage of whether to use “Username” or “User name” for the login.
According to dictionary.com, “username” is a word and it can also be used as [user name]. So both cases are correct and it only matters that you use it the same way consistently across a website.
“Dictionary.com Definition of username”
An example of using “Username” for its login is, Twitter (there are many other examples of this). Again, this is simple but I’ve run into the debate enough times I decided to settle the case.