What do you call a window that is neither opaque nor transparent?

Translucent is the appropriate word for the semitransparent material.

This type of glass is called frosted glass. As far as I know, there's no specific word for such a window. It's simply called a frosted glass window.

As Prakash already pointed out they're called frosted glass

To cover (glass, for example) with a roughened or speckled decorative surface.

From another site:

frosted glass diffuses light and blurs out the clear surface of the glass, making it ideal for bathrooms, entry doors and window panels.

On that site you'll see another word describing an opaque glass, too: milk glass. Milk glass is usually used

to make plates, goblets, serving dishes, jugs, pitchers and decorative items

but sometimes you'll hear of a "milk glass window". However it seems to me that it is not that much common using milk glass as frosted glass.

I think the word you are looking for may be translucent

OED: Allowing the passage of light, yet diffusing it so as not to render bodies lying beyond clearly visible; semi-transparent.

The previously-mentioned frosted glass is translucent, but that does not mean that all translucent glass is frosted.