Embedded Linux
Linux running on something that doesn't look like a computer — a router, a smart TV, a factory panel — with kernel and root filesystem cut to fit that exact hardware.
The border with microcontroller land runs roughly along the MMU. A chip with a memory management unit and a few tens of megabytes of RAM can run Linux; below that, an RTOS or bare metal takes over. The stack is nearly always the same: U-Boot brings up RAM and loads the kernel, a device tree tells it what hardware is present, and a minimal root filesystem built around BusyBox provides userland.
The whole thing is built with a cross-compiler, usually via Yocto or Buildroot. The payoff is ready-made drivers and a network stack; the price is boot times measured in seconds and an attack surface no vendor will patch for a decade. OpenWrt exists largely because router makers' own Linux builds stopped receiving updates the day after launch.