Ethmoid: 1. As an adjective: spongy or sievelike. From the Greek ethmos, sieve + eidos, resemblance = like a sieve. 2. As a noun; short for the ethmoid bone, a spongy bone that serves as the front floor of the skull and the roof of the nose. 3. As a noun: short for the ethmoid sinus, which is made up of numerous thin-walled air cells within the ethmoid bone.