What is Insane Delusion?

Legal Definition
Insane delusion is the legal term of art in the common law tradition used to describe a false conception of reality that a testator of a will adheres to against all reason and evidence to the contrary. A will made by a testator suffering from an insane delusion that affects the provisions made in the will may fail in whole or in part. Only the portion of the will caused by the insane delusion fails, including potentially the entire will. Will contests often involve claims that the testator was suffering from an insane delusion.

An insane delusion is distinct from testamentary capacity. A testator might be suffering from an insane delusion but otherwise possess the requisite capacity to make a will. Similarly, an insane delusion is distinct from a mere mistake. If suffering from an insane delusion, a testator is not subject to change his or her mind regarding the delusion if presented with contrary evidence, whereas a mistake is capable of being corrected if the testator is told the truth. Additionally, while an insane delusion may cause portions of a will to fail, most courts will not reform or invalidate a will because of a mistake unless it was the result of fraud.
-- Wikipedia
Legal Definition
Belief of facts which no reasonable person would have believed. See 63 Am. St. Rep. 83, note.
-- Ballentine's Law Dictionary