What is one of the features of a passport?

A passport is an official document issued by a government that certifies the identity and nationality of the holder for the purpose of international travel. One of its features is that it often serves as a primary form of identification when crossing borders. In contrast, a Seaman Discharge Book is specific to maritime workers and serves as proof of a sailor's employment and sea service, rather than being a general identity document like a passport.

The Visa Waiver Program and the Electronic Travel Authority are both travel programs that facilitate entry into certain countries without the need for a visa, but they are not features of a passport itself. Similarly, while birth certificates are important legal documents that establish identity and age, they do not serve the same international travel function as a passport.

Thus, understanding the role of a passport and distinguishing it from other documents is crucial in recognizing a passport's feature as a key identification tool in international travel.