A retroflex (ˈɹɛtɹoʊflɛks or ˈɹɛtɹəflɛks), apico-domal, or cacuminal (kæˈkjuːmᵻnəl) 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.
The Latin-derived word retroflex means "bent back"; some retroflex consonants are pronounced with the tongue fully curled back so that articulation involves the underside of the tongue tip (subapical). These sounds are sometimes described as "true" retroflex consonants. However, retroflexes are commonly taken to include other consonants having a similar place of articulation without such extreme curling of the tongue; these may be articulated with the tongue tip (apical) or the tongue blade (laminal).
Retroflex consonants, like other coronal consonants, come in several varieties, depending on the shape of the tongue. The tongue may be either flat or concave, or even with the tip curled back. The point of contact on the tongue may be with the tip (), with the blade (), or with the underside of the tongue (). The point of contact on the roof of the mouth may be with the alveolar ridge (), the area behind the alveolar ridge (), or the hard palate (). Finally, both sibilant ( or ) and nonsibilant (, , , ) consonants can have a retroflex articulation.
The greatest variety of combinations occurs with sibilants, because for them, small changes in tongue shape and position cause significant changes in the resulting sound. Retroflex sounds generally have a duller, lower-pitched sound than other alveolar or postalveolar consonants, especially the sibilants. The farther back the point of contact with the roof of the mouth, the more concave is the shape of the tongue, and the duller (lower pitched) is the sound, with subapical consonants being the most extreme.
The main combinations normally observed are:
Laminal post-alveolar, with a flat tongue. These occur, for example, in Polish cz, sz, ż (rz), dż.