Cloud Nine - Evanescence

Hey folks!

Actually, my doubt isn't about the song itself. It's about the title, I always see "Cloud Nine" translated as "Nas Nuvens", but for me, this is the translation for the whole song and there's nothing about the title. So finally, my question: Is there any other meaning for "nine" (that could make any sense together with "cloud")?

I thank you since now.

Do Michaelis: On cloud nine = Eufórico

Aaahhhh agora sim me faz sentido^^
Muitíssimo obrigada Vitor.