iso·cer·cal \| ̷ ̷ ̷ ̷|sərkəl\adjective Etymology:is- + -cercal 1.: having symmetrical upper and lower lobes and a gradually tapering vertebral column — used of the tail fin of a fish 2.: having or relating to an isocercal tail fin • iso·cer·cy\ˈ ̷ ̷ ̷ ̷ˌsərsē, -rkē\noun-es