The Electronic System for Travel Authorization (ESTA) is an online application system used by travelers from visa waiver countries to enter the United States. It is designed to enhance security and streamline the entry process for visitors. Travelers must complete the ESTA application before boarding a flight or cruise to the U.S., providing personal information and travel details. Once approved, an ESTA authorization is valid for two years or until the traveler's passport expires, whichever comes first. It allows multiple entries into the U.S. for stays of up to 90 days each. However, ESTA does not guarantee entry, as final admission is determined by Customs and Border Protection officials at the port of entry.