In software systems, encapsulation refers to the bundling of data with the mechanisms or methods that operate on the data. It may also refer to the limiting of direct access to some of that data, such as an object's components. Essentially, encapsulation prevents external code from being concerned with the internal workings of an object.

Encapsulation allows developers to present a consistent interface that is independent of its internal implementation. As one example, encapsulation can be used to hide the values or state of a structured data object inside a class. This prevents clients from directly accessing this information in a way that could expose hidden implementation details or violate state invariance maintained by the methods.

Encapsulation also encourages programmers to put all the code that is concerned with a certain set of data in the same class, which organizes it for easy comprehension by other programmers. Encapsulation is a technique that encourages decoupling.

All object-oriented programming (OOP) systems support encapsulation, but encapsulation is not unique to OOP. Implementations of abstract data types, modules, and libraries also offer encapsulation. The similarity has been explained by programming language theorists in terms of existential types.

In object-oriented programming languages, and other related fields, encapsulation refers to one of two related but distinct notions, and sometimes to the combination thereof:

Some programming language researchers and academics use the first meaning alone or in combination with the second as a distinguishing feature of object-oriented programming, while some programming languages that provide lexical closures view encapsulation as a feature of the language orthogonal to object orientation.

The second definition reflects that in many object-oriented languages, and other related fields, the components are not hidden automatically and this can be overridden. Thus, information hiding is defined as a separate notion by those who prefer the second definition.

The features of encapsulation are supported using classes in most object-oriented languages, although other alternatives also exist.