Thread
In plain terms
A lightweight unit of execution that shares its process's memory.
Going deeper
Threads run within a process and share its memory, making communication fast but requiring synchronization to avoid races. They enable concurrency within a single program.
Under the hood
Shares the parent process's address space, so it's cheap but requires synchronization; the GIL in CPython limits true parallel CPU-bound threading.
Examples
Worker threads, thread pools, Java Thread, pthreads, async runtimes.
Think of it like...
Like roommates sharing one apartment: efficient and close, but they must coordinate so two aren't using the kitchen the same way at once.