A
logical fallacy where something is
assumed to be
true because it hasn't been proven
false, or
vice versa. This is used very often, and
defended by insisting that the
counterargument must prove the premise
is false rather than prove that the premise
may be false.
An example: "No studies have proven that cigarettes cause cancer, so cigarettes don't cause cancer." (this may also be a case of False Premise)
To prove Argument from Ignorance, argue that the premise in question may be false/true even if not proven.
Note that this shouldn't be confused with Proof by Contradiction, where something is shown to be true/false by proving that assuming the opposite will lead to contradiction.