Extensible is an IT term used to describe something that can be extended or expanded from its initial state. It typically refers to software, such as a program or file format, though it can also be used to describe a programming language itself.
An extensible software program, for example, might support add-ons or plug-ins that add extra functionality to the program. It may also allow you to add custom functions or macros that perform specialized tasks within the application. An extensible file format (like XML) can be customized with user-defined elements.
If a programming language is extensible, it may support custom syntax and operations. These custom elements can be defined in the source code and are recognized by the compiler along with the pre-defined elements. Examples of extensible programming languages include Ruby, Lua, and XL.
Scalable vs. Extensible
While the terms scalable and extensible are sometimes used interchangeably, they are not the same thing. Scalability can refer to hardware, software, or an entire IT system, such as a cloud-based service. Extensibility, on the other hand, is almost always used to describe software and refers specifically to its extendable capabilities. For instance, a software program that supports plugins is extensible, but not necessarily scalable. A server rack that has several empty slots for future use may be considered scalable, but is not extensible.