Any mental disorder in which delusions play a significant role WordNet 3.0 & Open English WordNet

Delusional disorder, traditionally synonymous with paranoia, is a mental illness in which a person has delusions, but with no accompanying prominent hallucinations, thought disorder, mood disorder, or significant flattening of affect. Delusions are a specific symptom of psychosis. Wikipedia

Mental disease Wikidata

A mental disorder characterized by isolated delusions without any accompanying symptoms like hallucinations, thought disorder or flat affect. Wiktionary

Mental disorder. Wiktionary (translation)