A retroflex consonant is a coronal consonant where the tongue has a flat, concave, or even curled shape, and is articulated between the alveolar ridge and the hard palate. They are sometimes referred to as cerebral consonants, especially in Indology. Other terms occasionally encountered are domal and cacuminal.
In the International Phonetic Alphabet, retroflex consonants are indicated with a hook in the bottom right, such as [ʂ ʐ ʈ ɖ ɳ ɭ ɽ ɻ]. Alternatively, especially for those sounds with a relatively forward articulation (e.g. in the alveolar or postalveolar region rather than the hard palate), they can be indicated with a retracted diacritic (underbar); this occurs especially for [s̱ ẕ]. (Other sounds indicated this way, such as [ṉ ḻ ḏ], tend to refer to alveolo-palatal rather than retroflex consonants.)
Retroflex sounds need to be distinguished from other consonants made in the same parts of the mouth (postalveolar, alveolar, or palatal):
The first three types of sounds above have a convex tongue shape, which gives them an additional secondary articulation of palatalization. The last type has a groove running down the center line of the tongue, which gives it a strongly hissing quality. The retroflex sounds, however, have a flat or concave shape, with no associated palatalization, and no groove running down the tongue. The term "retroflex", in fact, literally means "bent back" (concave), although consonants with a flat tongue shape are commonly considered retroflex as well.