ethmoid n one of the eight bones of the cranium; a small bone filled with air spaces that forms part of the eye sockets and the nasal cavity Usage(s) These bony caverns are called the frontal, maxillary or antrum, ethmoid and sphenoid sinuses. They are the occipital, the two parietals, the frontal, the two temporals, the sphenoid (wedge-formed) and the ethmoid (sieve-formed). Synonym(s) ethmoid bone