retroflex vs retroflexion

retroflex is not a noun while retroflexion is a noun.

Word	Noun	Adjective	Verb	Adverb
retroflex	No	Yes	Yes	No
retroflexion	Yes	No	No	No

retroflex (noun)	retroflexion (noun)
	the act of bending backward
	an articulatory gesture made by turning the tip of the tongue back against the roof of the mouth
	a turning or tilting backward of an organ or body part

retroflex (adjective)	retroflexion (adjective)
bent or curved backward
pronounced with the tip of the tongue turned back toward the hard palate

retroflex (verb)	retroflexion (verb)
articulate (a consonant) with the tongue curled back against the palate
bend or turn backward