software that provides low-level control of computing device hardware. For a relatively simple device, firmware may perform all control, monitoring and data manipulation functionality.
https://en.wikipedia.org/wiki/Firmware
Bootloaders and BIOSes are often called firmware because they’re loaded into the flash memory of the processor itself.
from: https://itp.nyu.edu/physcomp/lessons/microcontrollers-the-basics/
from: https://itp.nyu.edu/physcomp/lessons/microcontrollers-the-basics/