I am confused. It seems like there are two definitions of "user" here: someone that uses something, and someone that has an account. It's possible to use Twitt

I am confused. It seems like there are two definitions of "user" here: someone that uses something, and someone that has an account. It's possible to use Twitter without an account (reading tweets seems like the primary use case of Twitter), and it's possible to have an account that you never use. People in the second case are counted as "inactive users" or, in other words, users that don't use Twitter. When people talk about the UX or UI of a website, they usually aren't talking about whether you have an account or not. It's possible for a website's UX to be bad before you even make an account, e.g. if the sign up page is bad.