firmware
also: BIOS, UEFI, device firmware
Low-level software that runs on hardware devices before the operating system loads, controlling the device's basic functions and hardware initialization.
Firmware is permanent or semi-permanent software programmed into hardware devices like network cards, SSDs, BIOS/UEFI chips, and routers. It sits between the hardware and the operating system, managing device initialization and core operations.
Unlike applications that run in userspace or kernel modules that load after boot, firmware executes directly on the device's processor. For example, your motherboard's UEFI firmware runs before Linux loads, initializing CPUs, memory, and storage devices so the bootloader can find and launch the kernel.
Firmware can sometimes be updated—Linux tools like fwupd help manage device firmware updates. However, updating firmware carries risk since a failed update can brick a device, making it unbootable.