noun
: one of the branches that arise from the ascending aorta in pairs in fishes and the embryos of higher vertebrates and pass to the gill clefts of either side and thence round the esophagus to fuse into the dorsal aorta and that are reduced in adult higher forms, one pair persisting in most amphibians and reptiles and a single arch, forming the base of the aortic trunk, in warm-blooded vertebrates, in birds the right and in mammals the left