cav

From Latin cavus 'hollow, empty,' and the related cavea 'a hollow, an enclosure, a cave.' Every cav- word centers on the idea of empty space inside something: cave (a natural hollow in the ground), cavity (a hollow space, including a hole in a tooth), excavate (to dig a hollow out), concave (curved inward, hollow on one side). Do not confuse with caut- (from cavēre 'to beware'), a different root.