Factuality Hallucination

Factuality hallucination occurs when an LLM’s output contradicts verifiable real-world facts or cannot be verified against established knowledge.

This divides into two subtypes:

Factual Contradiction: Output contains facts that can be checked against reality and are demonstrably wrong. Includes entity errors (wrong people, places, things) and relation errors (correct entities, wrong relationships between them).

Factual Fabrication: Output contains claims that cannot be verified, either entirely made up (unverifiability) or stated with false certainty when reasonable disagreement exists (overclaim).

The distinction matters for mitigation: contradictions can potentially be caught by fact-checking against knowledge bases, while fabrications require different approaches since there’s nothing to check them against.

Related: 05-atom—faithfulness-hallucination-definition, 05-atom—knowledge-boundary-problem, 05-molecule—llm-hallucination-taxonomy