branchial cleft

noun

: one of the open or potentially open clefts that occur on each side of the neck region of a vertebrate embryo between the branchial arches, are formed by the meeting of an external furrow in the ectoderm with a pharyngeal pouch, may or may not extend through from the exterior to the cavity of the mouth and pharynx, and correspond to the gill slits of fishes and amphibians

called also pharyngeal cleft, pharyngeal slit, visceral cleft