1. to work or operate in a proper or particular way.
"her liver is functioning normally"
synonyms:work, go, run, be in working/running order, operate, be operative
"the electrical system had ceased to function"
An exercise labeled as "functional" that biomechanically violates your true human function is no longer so based on it's definition.
An exercise labeled "non-functional" that improves the quality of your muscular system and increases joint integrity is now by definition very functional.