Encapsulation is one of the fundamental concepts of object-oriented programming (OOP) that refers to the binding of data and code together in a single unit called a class. It is a mechanism of hiding the internal state and behavior of an object from other objects or external entities while allowing controlled access to the exposed data and methods.

In simpler terms, encapsulation is the process of wrapping data (attributes/fields) and functions (methods/behaviors) within a class, which helps in protecting from unauthorized access or unwanted modifications. It is a technique that provides data and behavior security to the class and objects.

In Java, encapsulation is achieved through the use of access modifiers such as private, public, protected, and default (package-private) that determine the visibility of the data and methods. The private modifier ensures that the data and methods are only accessible within the class, while the public modifier allows them to be accessed from any other class. The protected modifier enables the members to be accessed within the class hierarchy and package-private modifier allows access within the package.

Encapsulation is an important characteristic of OOP as it simplifies the usage of classes by providing a clear interface to its behavior and prevents unauthorized changes to its internal state, thus ensuring stability and reliability.