(a.) Middle; middlemost.

amidships average axial center centermost central centralized centric centroidal centrosymmetric concentrated core diameter diaphragm equator equatorial equidistant geocentric halfway heart interior intermediary intermediate kernel key mean medial median mediocre mediterranean medium mesial mezzo mid middle middlemost middling midland midriff midships midst midway nuclear nucleus omphalic pivotal thick thick of things umbilical waist waistline zone