Open a codebase you've never seen before and try to figure out what it does. Sometimes you stare at imports, configuration, error handling, and connection pools for an hour before you find the file where something actually happens : the file where money moves, an order ships, a patient gets scheduled. That file is the domain. Everything else is plumbing. The domain is the part of your software that talks about the problem you're solving, in the terms of the people who have the problem. A banking system's domain is accounts, transactions, balances, transfers, interest accrual, overdrafts. A clinic's domain is patients, appointments, providers, visits, prescriptions, claims. A logistics company's domain is routes, shipments, drivers, manifests, exceptions. The domain has nothing to do with whether the database is Postgres or DynamoDB, whether the API is REST or gRPC, whether the deploy target is Kubernetes or a single VM. Those are real engineering decisions and they matter, but they aren't the thing .…