What is the Global Interpreter Lock (GIL) in Python?

GIL is a mutex in CPython that allows only one thread to execute Python bytecode at a time, even on multi-core systems. Purpose: simplifies memory management, makes C extensions easier. Impact: CPU-bound tasks don't benefit from multi-threading. Solutions: multiprocessing (separate processes, no GIL), use C extensions that release GIL, asyncio for I/O-bound tasks, alternative implementations (Jython, PyPy). I/O-bound code releases GIL during I/O operations, so threading works well. For CPU-bound parallelism, use multiprocessing or ProcessPoolExecutor.