Look for term/name about non-trivial programming language extentions

Look for term/name about non-trivial programming language extentions

Such things like:
alloc/free, pthread_*, longjump in Ñ
Thread, ThreadLocal, ClassLoader, JavaCompiler, java.lang.instrument,
WeakReference etc in Java
C++ template language extension (back in the past)
runtime binary/byte code instrumentation
require especial support from runtime and add new semantic to language
which can't be expressed by language itself.
Also assembler language extension or external modules or build-in
reflection API have access to runtime and can change behavior.
I look for term that give name for such language extension in general
which bring new semantic to language that can't be expressed by usual
language constructs.