RIOT powers the Internet of Things like Linux powers the Internet. RIOT is a free, open source operating system developed by a grassroots community gathering companies, academia, and hobbyists, distributed all around the world.
RIOT supports most low-power IoT devices and microcontroller architectures (32-bit, 16-bit, 8-bit). RIOT aims to implement all relevant open standards supporting an Internet of Things that is connected, secure, durable & privacy-friendly.
Back in my day, we used to call this an embedded operating system.