An object oriented programming concept from the Java programming language. Essentially you create a class inside of another class. The inner and outer class both have a heightened level of access each to each other, and the inner class is only accessible from the outer. An example:
class Outer
{ int whatever;
class Inner
{
public Inner() { System.out.println("Inner class.");
}
Inner i = new Inner();
}
You get the idea.