What is the rear gears called on a bike?
Depends on the bike. If the bike is a single speed or possibly fixed gear bike, there's only one gear on the rear. The bike could also be equipped with an internal hub system that LOOKS LIKE one gear, but could have anywhere from 3 to 8 gears. https://en.wikipedia.org/wiki/Hub_gear If the bike has somewhere between 5 to 7 gears on the rear, it's most likely a "freewheel" design. This is usually found on older bikes and/or cheap ones. Better bikes now use what's called a "freehub & cassette". This could be 7 rear gears - but most likely 8 or more up to 11. A freewheel simply screws onto the hub. A cassette slides onto a spline. This link explains it all very well. https://www.sheldonbrown.com/free-k7.html
Which bike? As others have mentioned, there are several different types of gearing systems If you want specific names, you have to tell people which bike you are asking about
If there's just one, it's a cog or a sprocket. If there's several different size ones piled up, it's a cluster, block, or cassette.
Cog(s) or sprocket(s); these are then designed to be put on a: internal geared hub, fixed hub, flip flop hub, coaster hub, freewheel hub or a cassette hub. The front ones at the crank end are called (a) chainring(s).
Bikes don’t go backwards!
Sturmey Archer hub, rear cog for Derailleur.
Derailleurs? (I'm not sure.)