Call application extension programming language (AEPL) a programming language designed to be allow complex user input, to describe complex courses of action to be taken, to easily extend available application logic.
Yet, unlike a common programming language, an application extension programming language is not design to just allow. An AEPL script or program will not be executed by an interpreter that has no business but to execute that script or program, but by an application that has otherwise a full agenda and is quite not into messing up its users poor existence more than it already does without running any script or program. That is to say that the execution of a script or program has got to be very safe with respect to just about everything: memory allocation, length of execution, environment pollution.
And again unlike a programming language designed to write applications and not to extend them, an application extension programming language does not necessarily have to be very complex nor should there be a need for a very fast interpreter for it (not that it otherwise wouldn't be desirable to have available such a very fast interpreter).
Useful application extension programming languages are only as useful as their interpreters are. An AEPL interpreter should be as friendly and forthcoming toward application just as the application extension programming language itself is friendly and forthcomming toward user (the programmer). And an AEPL should be as well very reassuring: it has to allow the application to control resources management, including memory, procession power and anything else that may apply.