Ubiquitous Language

DDD's first move is to share vocabulary between domain experts and developers. If the business says invoice, the code says Invoice. If the business distinguishes invoice from receipt, the code does too — and the model stops drifting from the business.

The ubiquitous language is the contract that keeps the model honest.

When pilots and air-traffic controllers say "affirm" instead of "yes", it isn't pedantry — it removes ambiguity in a high-stakes channel. Ubiquitous language is the same idea, applied to your domain.