What does a valid, unexpired U.S. passport serve as?

A valid, unexpired U.S. passport serves as proof of identity. This document is issued by the U.S. government and contains personal information such as the holder's name, date of birth, and photograph, making it a reliable form of identification. It is often accepted in various situations that require the verification of an individual's identity, such as when applying for a driver's license or entering federal buildings.

While other documents may serve different purposes, a passport is specifically designed to confirm the identity of the bearer, which is why it is widely recognized as a valid form of identification both domestically and internationally.