Is there a special term for refractive indices when 1 is subtracted?

I have been doing a bit of studying for my optics course and I have often come across refractive indices or materials being presented as n-1 (which often makes a lot of sense). I was just wondering if there was a special term like for this format or if we still call it just the "refractive index"?

Sorry if this is in the wrong section, it's not directly for homework although I will probably use it for homework. It's more just for my own information.

thanks!