Having an empty space or cavity, natural or artificial, within a solid substance; not solid; excavated in the interior; as, a hollow tree; a hollow sphere.(adjective)
source: webster1913
interjection (int)
Hollo.(interjection)
source: webster1913
noun (n)
a cavity or space in something(noun.shape)
Example: Hunger had caused the hollows in their cheeks.
source: wordnet30
a small valley between mountains(noun.object)
Example: He built himself a cabin in a hollow high up in the Appalachians.