Actually, Ametsuchi no Uta (Uta means a song) is not a song and it may be appropriate to be called as Ametsuchi no Kotoba (literally, words of heaven and earth), however, it is commonly called as Ametsuchi no Uta.
実態から言えば歌ではないので、天地の詞（ことば）という呼び方の方が適切であるが、天地の歌で通用している。