Appium is an open-source automation framework designed for testing native, hybrid, and mobile web applications across iOS, Android, and Flutter platforms. Built on a client-server architecture using the WebDriver protocol, Appium enables cross-platform testing with a single, unified API, making it the most widely adopted mobile automation tool in 2026. Its modular, driver-based ecosystem introduced in Appium 2.0 allows teams to install only the drivers they need—and the fact that it's completely free makes it accessible for teams of any size. One critical insight: successful Appium automation depends heavily on choosing stable locators and proper synchronization—flaky tests are almost always caused by fragile locators or missing waits, not Appium itself.