Definition: An appeal to ignorance uses lack of evidence (for or against) as the basis of the argument. For example, if something can’t be disproven, it must be true!
Example: You have a family member that has a terminal disease. You hear of a possible new cure being offered in another country. You contact the group promoting this cure and ask if it works. They say, “No one has ever shown that it doesn't work, so of course it works!”