pectoral fin
noun [ C ] biology specialized uk/ˈpek.tər.əl ˌfɪn/ us/ˈpek.tɔːr.əl ˌfɪn/
胸鳍（鱼身体前面两侧的鳍，用于控制和减速）
the fin on each side of a fish near the front of its body, used for controlling the direction the fish goes in and for slowing it down