Driver
Software component that lets the OS talk to specific hardware. Translates between generic API calls ("read sector X") and the hardware's protocol.
Runs in kernel space (ring 0) → bugs = blue screen / kernel panic. Linux has drivers integrated into the kernel (40M+ lines of code is drivers); Windows keeps them separate. NVIDIA drivers infamously full of proprietary code → Linus Torvalds's finger photo, 2012. Modern alternatives: user space drivers (DPDK for networking, SPDK for storage), eBPF (safe "drivers"), microkernels that separate drivers from the kernel entirely.