Representational State Transfer Application Programming Interface - a standardized way for software applications to communicate over the internet using HTTP methods like GET, POST, PUT, and DELETE. Like a universal language that lets different programs request and share data with each other.
A mobile weather app uses a REST API to fetch current conditions from a weather service by sending a GET request to an endpoint like /api/weather/london and receiving JSON data in return.
REST APIs are an architectural style, not a single cloud service. These managed API gateway/API management services commonly publish, secure, throttle, monitor, and route REST endpoints.