aer

Aer, aeris, m. A loan from Greek, and it shows: the noun keeps a Greek accusative aera beside the regular Latin aerem, and it is masculine despite a nominative that looks neuter. Both accusatives are common, so learn the pair together.

Classically aer is the lower air, the misty stuff near the ground that weather happens in; the bright upper air is aether, and the two are not interchangeable in poetry. Watch the genitive: aeris is also the genitive of aes, bronze. Italian mal aria, bad air, is how English got malaria, back when the disease was blamed on marsh vapours rather than mosquitoes, and the same root gives aerial, aeroplane and aerobic.