A monitor is a synchronization construct that guarantees mutual exclusion for a block of code while also providing a mechanism for threads to wait until a condition becomes true. Originally formalized by Tony Hoare and Per Brinch Hansen in the 1970s, the monitor abstracts over low-level primitives like semaphores by bundling the lock, the data it protects, and the condition variables into a single linguistic construct. Every object in Java is implicitly a monitor: the "synchronized" keyword compiles to monitor-enter and monitor-exit bytecode instructions, and the Object.wait() / Object.notify() methods expose condition variable semantics. The monitor is the foundational synchronization primitive of the Java HotSpot VM and many other managed runtimes, though modern implementations decompose it into lighter-weight structures like biased locks and thin locks to avoid the overhead of full monitor escalation.
See also: Concurrency, Lock Elision, Java HotSpot VM
Related: Semaphore, Condition Variable, Mutex