AngularJS is best described as an open-source JavaScript framework designed for creating single-page web applications (SPAs). It enables developers to use HTML as a template language, extending HTML's syntax to express application components clearly and succinctly. AngularJS is renowned for its data binding and dependency injection features, which significantly reduce the amount of code developers need to write. The framework operates entirely within the browser, making it a versatile choice for developing dynamic and interactive web applications in conjunction with any server-side technology. AngularJS follows the Model-View-Controller (MVC) architecture, aiming to simplify both the development and testing of web applications by providing a structured and modular framework.