Java polymorphism is the concept that allows objects of different classes to be treated as objects of a common superclass. It is a fundamental principle of object-oriented programming that enables one interface to be used for a general class of actions. The two types of polymorphism in Java are compile-time (or method overloading) and runtime (or method overriding). Compile-time polymorphism allows method resolution during compile time, whereas runtime polymorphism resolves method calls at runtime, enabling objects to behave differently depending on their actual class type. This flexibility makes code more reusable and easier to maintain.