Definition
Plain language
The low-level software baked into a piece of hardware, below the operating system.
As stated in the literature
Code resident in non-volatile device memory (BIOS/UEFI, BMC images, device controllers) that runs before and beneath the OS; persistence at this layer survives disk reimaging.
Why it matters: Anything hiding at this level runs before the operating system and its defences exist, so it survives reinstalling the system and is very hard to detect or remove.
For example, the code that draws the logo and checks your hardware in the seconds before Windows or Linux starts loading is firmware.
Heard on the show
“Then, in a separate experiment, they gave a model the engine name and a real bug for it, and walked a proof-of-concept from a chat window, down toward the firmware on the motherboard.”Episode 272 — How a Model Guesses Which Engine Is Running It, From a Wrong Date