Expedia API refers to all the developer APIs that Expedia provides. This includes:
Some may find that getting access and using the Expedia API could be troublesome. In this API collection, you'll find tons of easy to use REST APIs that you can use instead. These APIs listed here provide data and functionality on booking, travel, car rental, and more.
GDS API Integration. GDS is a worldwide "Global Distribution System" computerized reservation network used as a single point of access for accommodating airline seats, hotel rooms, rental cars, and other travel related items by travel agents, online reservation sites, and large corporations. - Source
Yes, TripAdvisor has an API, which you can access here. Per the API's description: These APIs help query real-time flight prices, hotel bookings, restaurants, attractions, and more.
Yes, but as of April 10, 2018, the QPX Express API has been deprecated. While this API is deprecated, there are still many flight APIs available including Skyscanner, which is widely accepted as an alternative to the Google Flights API.