What is ESTA?
ESTA meaning Electronic System for Travel Authorization (ESTA) is an automated system that determines the eligibility of visitors (nationals from 38 participating countries) to travel to the United States without a visa under the Visa Waiver Program (VWP). A valid ESTA approval is required for all VWP travel to the United States. ESTA applications may be submitted at any time prior to travel, though it is recommended travelers apply when they begin preparing travel plans. To learn whether you may be able to travel on the VWP, and therefore whether you need an ESTA authorization, see our Visa Waiver Program webpage. For more information about ESTA and/or to apply, see the DHS, Custom and Border Protection’s ESTA website.
reference: U.S. Visas – Glossary