A programming language. like Python, that can be used to extend the functionality of programmable languages or interfaces.

a programming language interpreter offered by an application program, so that users can write macros or even full-fledged programs to extend the original application

a programming language that you can use to extend the functionality of a large appli- cation

a (usually) interpreted language built into a program that allows some behaviors of the program to be modified even after it has been compiled and distributed

Extension languages are programming languages that allow users, programmers, and third-party developers to add features to a program without having to re-write the program as a whole, and it allows people extending a program to co-operate with each other, without having to expend any extra effort.