In object-oriented programming, the open/closed principle states "software entities (classes, modules, functions, etc.) should be open for extension, but closed for modification";[1] that is, such an entity can allow its behaviour to be extended without modifying its source code.

The name open/closed principle has been used in two ways. Both ways use inheritance to resolve the apparent dilemma, but the goals, techniques, and results are different.

Bertrand Meyer is generally credited for having originated the term open/closed principle,[2] which appeared in his 1988 book Object Oriented Software Construction.

A module will be said to be open if it is still available for extension. For example, it should be possible to add fields to the data structures it contains, or new elements to the set of functions it performs.
A module will be said to be closed if it is available for use by other modules. This assumes that the module has been given a well-defined, stable description (the interface in the sense of information hiding).[3]

--https://en.wikipedia.org/wiki/Open/closed_principle


You should be able to extend a classes behavior, without modifying it.
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