Hallucination
Also: confabulation
Output that sounds plausible but is fabricated: invented facts, citations, dates or names, or claims absent from a provided document. It arises because models generate likely text rather than verified facts; grounding with retrieval and checking claims against sources reduces it.
Example
An abstractive summarizer writes "the policy took effect in March 2021" although no date appears in the article. ROUGE still rewards the overlapping words, so the error goes unnoticed without a faithfulness check.