Hi and welcome to NICO Club!
Coilover is a term that can be used accurately and loosely. Your understanding of coilover is accurate. Some people loosely refer to shock and spring sets as coilovers when they are indeed not. Maybe because the front ones are and the rear aren't, I'm not sure why.
For your car, yes, there are kits that are called coilover kits and they can have the separate spring and shock for the rear. Additionally, there are kits called coilover kits, also for your car, and they can have a true coilover in the rear. The spring bucket sits empty and unused.
Here's a neat thread that goes over a bunch of options for the Altima.
coilover-lowering-spring-options-t559823.html
You'll notice discussion of the D2 setup and how that is a true coilover in the rear.
I hope that helps!