noun

an empty space within a solid object.

the abdominal cavity

space,chamber,hollow,hole,pocket,pouch,orifice,aperture,socket,gap,crater,pit,cutting,concavity,lacuna,sac,alveolus,ampulla,antrum,archenteron,bulla,bursa,lumen,orbit,sinus,ventricle,vesicle,calyx,cyst,geode,vug