Is there a purpose for stretching other than flexibility? Is there a way to achieve flexibility other than stretching?
Both of these tags have empty wiki entries and are rarely used. I propose making them tag synonyms.
My impulse is that flexibility is about increasing the range of motion whereas stretching involves the warming up on the muscles and tendons, which can be used to increase flexibility, but might also be used to simple avoid injury within the already-achieved range of motion.
Since this is martial arts site, I'd probably say that
stretching has a potential ambiguity with submission stretches, but that's probably addressable on a case by case basis.