It's a rear cassette to be more precise
Definition of cassette: 1. small case or cartridge, 2. a lightweight case or cartridge containing an interchangeable medium. It's not a cassette because it is not a container.
You MAMILs are so pretentious, you're getting to be like yachties - it's a secondary gear cluster. If you call it what it is even normal people will know what you mean.
When I used to sail my friends and I made a point of avoiding nautical terms. So 'mooring rope' became 'parking string', the 'genoa' became 'the front sheet', the 'mainsail' the 'big sheet', the 'mainsheet' (which is actually a rope) became the 'big string', the 'cockpit' was the 'patio', the 'saloon' the 'lounge', the 'cabins' 'bedrooms', the 'bow' was called the 'pointy end', the 'main hatch' the 'front door'. A much more inclusive and descriptive set of terms
Mooring, or parking as we called it, was always hilarious for the reactions of the sailing cognoscenti.
Well, it used to cheer us up in force 8. And in bars when the other 'serious' yachties would look at us like we were from another planet