[n] an ornamental sunken panel in a ceiling or dome [n] a blank gap or missing part
blank, caisson, coffer
alveolation, alveolus, antrum, aperture, armpit, arrearage, basin, bowl, breach, break, broaching, caesura, cavity, cessation, chasm, check, clearance, clearing, cleft, concave, concavity, crack, crater, crypt, cup, defalcation, defect, deficiency, defici