As I'm making some research on DeFi I got to Werner et al. (2021). In there a lot of terms around DeFi are explained, but the actual meaning of a protocol as the core of a DeFi project itself is not explained. Even in other Threads such as What is DeFi? [Ref:StackExchange] a clear definition is missing.
So as all DeFi projects themselves are quite differently structured I wonder how to define a protocol (if it is the umbrella term for all DeFi projects).
Looking forward your responses!