What is the appeal to ignorance / burden of proof fallacy (ad ignorantiam)?

Taking the absence of evidence against a claim as proof the claim is true (or vice versa), or dumping the burden of proof onto the wrong side.

The pattern: "No one has proven X is false, therefore X is true." It fails because not having disproved something is not the same as having established it — and in most cases the burden of proof lies on whoever asserts that something exists or is the case.

"You can't prove that ghosts don't exist, so they must be real."

The lack of a disproof is not evidence for ghosts; the claimant has to provide positive evidence. The burden normally falls on the person making the positive existence claim (Bigfoot, psychic powers, an undetectable teapot) — and "innocent until proven guilty" is a deliberate, codified placement of that burden.

Tip: Slogan to remember: "Absence of evidence is not evidence of absence" — and equally, absence of disproof is not proof.