What does “cassette” mean in a stamp auction description?

Terri (philatelia ), I think it is referring to a slip-cover or slip-case for an album.

It looks to me like a term used by non-native English speakers.

I am not using a dictionary, I am taking the meaning from context.

On the auction website: ".... album plus cassette... "

So, I think it means a slip-case.

"Cassette" is a trick word, which has many meanings depending on context. In old French, it used to mean a small wooden box (like where you would keep jewelry). The meaning has thus evolved to extend to lots of small containers. In the music industry, it's the "cassette (K7) magnetic tape", so "album plus cassette" could be an old vinyl album accompanied by the cassette tape holding the same music. I am unaware of the word having a special philatelic meaning, so if the auction item is stamp related, then I would lean towards an old small container box of some kind.

Ah, OK. With that example, I would understand the protective cardboard box slip cover that surrounds the albums to help them stand vertically on a shelf.