Which Authentication category does a Username and Password fall under?

A username and password fall under the "something you know" category in authentication. This category is part of the knowledge-based authentication method, where the user proves their identity by providing only information they are supposed to know—typically, a unique combination of a username and password.

This form of authentication is widely used and is the most basic type of security. However, relying solely on "something you know" can be vulnerable to attacks such as phishing, brute-force, and credential theft. For stronger security, username and password authentication are often combined with additional factors, like "something you have" (e.g., hardware tokens) or "something you are" (e.g., biometrics), forming multi-factor authentication (MFA) systems.