IDEAS OF REFERENCE are
overvalued ideas in which the patient is virtually,
but not totally, convinced that objects, people, or events in his immediate environment have personal significance for him. For example, whenever a man sees a red car parked in front of his house, he is convinced that it is a "message" from the FBI warning him to be on the alert for drug addicts.
These are similar to delusions, but vary in belief solidity: a delusion is a false belief about external reality that is firmly maintained despite contrary evidence or proof.