Bare metal
Code that runs directly on the processor with no operating system in between.
In embedded work it means the program sets up its own stack, clocks and interrupt vectors, then drops into an endless main loop — a superloop that polls sensors and reacts to flags set by interrupt handlers. No threads, no memory protection, no scheduler. Anything not done in the loop or in an interrupt doesn't get done.
The upside is total control and almost zero overhead: an 8-bit microcontroller with 2 KB of RAM has no room for anything else. The downside shows up when requirements grow and three things with different deadlines need to happen at once — which is usually when an RTOS appears. In the cloud world the phrase has picked up a second meaning: a rented physical server with no hypervisor.