A user authenticated to more than one website using their same credentials is an example of:

Multifactor authentication requires two or more distinct verification factors (e.g., password plus a token) for a single authentication event, not access to multiple sites with one credential.

User access control refers to permissions and authorization policies that define what resources a user can access, not the authentication method used across sites.

Two-factor authentication is a specific form of multifactor authentication requiring exactly two factors for one login, not a mechanism for reusing credentials across multiple sites.

SSO is an authentication scheme where a user logs in with a single set of credentials and is then automatically authenticated to multiple applications or websites within a federated trust relationship. This matches the scenario described - one credential set granting access to more than one site - and reduces password fatigue while centralizing authentication management.