A user often has a user account and is identified to the system by a username or user name.
Other terms for username include login name, screenname or screen name, nickname or nic) and handle, which is derived from the identical Citizen's Band radio term.
In terms of this website that you currently view, a user's account allows a user to authenticate to the system and potentially receive authorisation to access resources provided by or connected to that system.
However, authentication does not imply authorization. To log in to an account on this wiki, a user is typically required to authenticate herself with a password or other credentials for the purposes of accounting, security, logging, and resource management.