adj

• (phonetics) Of pronunciation in which the tip of the tongue is raised and bent backwards, so that the underside of the tongue approaches or touches the palate.
  "In the case of the Wubuy alveolar versus retroflex contrast for instance, we argue that the phonetic overlap is so great that native listeners can behave as if this were an instance of a good and a 'less good' instance of a single category (CG)."
• (phonetics) Of pronunciation in which the tip of the tongue approaches or touches the back of the alveolar ridge.
• (phonetics) Of pronunciation in which the blade of the tongue approaches or touches the back of the alveolar ridge.
• (phonetics, general sense) Of any of the aforementioned pronunciations.

noun

• (phonetics) A consonant pronounced with the underside of the tongue approaching or touching the palate.
• (phonetics) A consonant pronounced with the tip of the tongue approaching or touching the back of the alveolar ridge.
• (phonetics) A consonant pronounced with the blade of the tongue approaching or touching the back of the alveolar ridge.
• (phonetics, general sense) Any of the aforementioned consonants.

verb

• (transitive) To bend or curve backwards.
  "[…] retroflexing the endoscope tip and mechanically rotating the transducer."