Mutex
In plain terms
A lock that ensures only one thread touches shared data at a time.
Going deeper
A mutex ('mutual exclusion') is acquired before entering a critical section and released after, serializing access so concurrent threads don't corrupt shared state. Misuse can cause deadlocks or slowdowns.
Under the hood
Blocking and potentially costly under contention; alternatives include read-write locks, spinlocks for short critical sections, and lock-free structures using atomic CAS.
Examples
pthread_mutex, Java synchronized, Go sync.Mutex, std::mutex in C++.
Think of it like...
Like the single key to a one-person restroom: you must hold the key to enter, and others wait until you hang it back up.