A passport number is a unique identifier assigned to an individual's passport, which is an official document issued by a government. This number helps to distinguish one passport from another and is essential for tracking and verifying the authenticity of the document. It typically consists of a combination of letters and numbers. When traveling internationally, the passport number is required for various processes, such as booking flights, applying for visas, and passing through customs. It is important to keep this number secure, as it can be used for identity theft if it falls into the wrong hands.