groove (n.)

A term sometimes used in PHONETICS to refer to a type of FRICATIVE where the TONGUE is slightly hollowed (or grooved) along its central line, the passage of air producing a sound with a higher frequency than in other fricatives. In English, [s], [z],  and  are ‘groove fricatives’. In SLIT (or ‘flat’) fricatives (e.g. [f], [θ]), there is no such groove (or grooving).