What's a monitor in Java?

A monitor is mechanism to control concurrent access to an object.

This allows you to do:

Thread 1:

public void a()
{
    synchronized(someObject) {
        // do something (1)
    }
}

Thread 2:

public void b()
{
    synchronized(someObject) {
        // do something else (2)
    }
}

This prevents Threads 1 and 2 accessing the monitored (synchronized) section at the same time. One will start, and monitor will prevent the other from accessing the region before the first one finishes.

It's not a special object. It's synchronization mechanism placed at class hierarchy root: java.lang.Object.

There are also wait and notify methods that will also use object's monitor to communication among different threads.

A monitor is an entity that possesses both a lock and a wait set. In Java, any Object can serve as a monitor.

For a detailed explanation of how monitors work in Java, I recommend reading the Monitor Mechanics section of Concurrent Programming in Java (the preceding link displays the preview in Google books, and that section is available for reading).

1. A monitor is a concept/mechanism that's not limited to the Java Language;
2. "In concurrent programming, a monitor is an object or module intended to be used safely by more than one thread";
3. As every reader knows, every object in Java is a sub-class of java.lang.Object. The java folks made java.lang.Object in such a way that it has features and characteristics that enables Java programmers to use any object as a monitor. For example, every object has a wait queue,a re-entrance queue and wait and notify methods making it a monitor;
4. read about monitors here.