Tying every claim in an answer to the source it came from, and showing the user. It is the main defence against invented answers, because a claim with no source is visibly unsupported, and it turns the system from something you have to trust into something you can check.
Technical view
Implemented by requiring span-level or chunk-level references in the output, validated post-generation by checking that cited sources exist and support the claim, often with a separate entailment check. Failure modes include citing a real source that does not contain the claim, and correct claims with no citation. Attribution should be evaluated explicitly rather than assumed from the presence of citation markers.
Related
Part of AI Terminology Index, section: Retrieval and Knowledge.