In normal photography use, "opaque" is not the correct term for a flash diffuser. A diffuser is meant to let light pass through while scattering it, so the proper term is usually "translucent."

"Opaque" means light does not pass through, which would make it a poor diffuser. Some sellers may use "opaque" loosely to mean you can’t see clearly through the material, like frosted glass, but that’s still technically better described as translucent.

"Semi-opaque" isn’t a standard or especially precise term here either. If a diffuser passes some light and scatters it, "translucent" is the clearest description.

In unusual technical cases, a material could be opaque to certain wavelengths while passing others, but that’s probably not what typical flash diffuser listings mean.