Measurement
First-party data
First-party data is information a business collects directly from its own customers and its own properties, rather than obtaining it from a third-party source.
Its importance grew as third-party cookies and mobile identifiers became unreliable, leaving purchase history, email engagement and on-site behaviour as the durable signals a brand actually owns.
The practical requirement is that it be joinable. Order data in one system, email engagement in another and site behaviour in a third are three datasets, not first-party data — the value only appears when a customer can be followed across all three.