OData
What it is
The REST-based protocol SAP uses to expose business data and operations to UIs and integrations. RAP business objects are published as OData (V2 or V4) services; Fiori apps consume them.
Why it matters for Clean Core
OData is the contract between your RAP service and its consumers. In RAP you rarely hand-write it — the framework generates the OData service from the model and behavior — but understanding it helps you debug and design services.
Common pitfall
Hand-building gateway/OData like in the SEGW days. With RAP, the service is published from the model; reaching for classic gateway service development for new work is doing by hand what RAP does declaratively.