A username identifies an account within a particular service or application. It is normally used alongside another authentication factor, such as a password, and is separate from a device's local network name. The same username text can exist in different systems without referring to the same account, provider or set of subscription permissions.
A practical example
A customer may have one username for a player publisher's website and another for the streaming account opened inside that player. Entering the publisher login into the media service's fields will not make the two independent account systems interchangeable.
A common misunderstanding
Do not assume that an email address or visible profile name is the issued media username. Use the account details and login format actually supplied for the relevant service.
Related terms
Stable technical definition. Editorial approach