What is a unique identifier for a user that allows them to access systems or services?

A username serves as a unique identifier for a user, granting them access to various systems or services. It is often a distinctive name or label that represents the individual within a platform, allowing the system to recognize and differentiate between users.

In a typical access scenario, a username is paired with another piece of information, such as a password or PIN, to ensure security and verify the identity of the user. While options like password, PIN, and token are all components of secure access methods, they serve different functions. For instance, a password is a secret word or phrase that, in conjunction with a username, authenticates the user, while a token may involve a physical device or software application that generates a code to further secure access. However, it is the username that fundamentally identifies the user in the system, making it pivotal for access.