Based on this answer: https://ethereum.stackexchange.com/a/37419/24310
How does interest work with casper? Is it correct to describe what the validators receive as interest?
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Answering a question that links to my own answer on another question, ha!
So there are really two flavors of Casper, one of which is hybrid PoS/PoW and the other is full Pos.
Casper FFG (Friendly Finality Gadget): This is the hybrid that runs PoS as a contract on top of the existing PoW system in order to achieve finality so block history revision is less effective. In this system, the economics (you can read the details here) work out to pretty accurately be called interest. Essentially, there exists a global interest based on how the whole system is running, and a per-validator interest based on how each actor is running. All validators are paid a percentage based on the global/per-validator rates on a per-epoch (50 blocks) for voting.
Casper TFG (The Friendly Ghost): Also often called CBC (Correct by Construction), but that's actually a family of Casper protocols that it belongs in. The economics of this are either not flushed out or just not published yet AFAIK, but it'll likely work on a similar percentage basis based on how well the system is running and how well each validator is running. More on the mechanics, but not really the economics, can be read here