What is a Hypoid Gear?

[chitowndad]

I was just about to hit the submit button to pickup 3 qts of RedLine MT90 when I read that it said not to be used on Hypoid Gear Transmission, what the heck is that?

Here's the text "MTL, MT-85 & MT-90 are not for use in differentials with hypoid gears"

here's the link on the MT90 specs
Gear Oil and Transmission Lube - Red Line Oil MT-90 Manual Transmission Lube

any insight is appreciated.

Thanks,
-Steve