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Noun[edit] cavity â(plural cavities) 1  A hole or hollow depression. 2  A hollow area within the body (such as the sinuses). ( dentistry) A soft area in a decayed tooth.
Definition of hollow. 1  1 : an unfilled space : cavity, hole in the hollow of a tree. 2  2 : a depressed or low part of a surface; especially : a small valley or basin.