Argumentum ad ignorantiam means "argument from
ignorance". This
logical fallacy occurs when it's argued that something must be
true, simply because it hasn't been
proved false. Or, equivalently, when it is argued
that something must be false because it hasn't been
proved true.
(Note that this isn't the same as
assuming something is
false until it has been proved true. In
law, for example, you're generally assumed
innocent until proven
guilty.)