Karl Popper defines an ad hoc hypothesis as one that is introduced to immunize a theory from some (or all) refutation but which cannot be tested independently.
Central to Popper's account of science are the claims that theories can avoid or be immunized from refutation and that this practice is, in general, unscientific, or at least of dubious scientific value.(1) One important means of avoiding a refutation or immunizing a theory from refutation, according to Popper, is by the introduction of an ad hoc hypothesis. For Popper ([1974b], p.
287) asserts or implies that it is ad hoc.(10) But as we have seen, an ad hoc hypothesis is one that, according to Popper, is not independently testable of its explanandum, and it does not follow that a replacement hypothesis with less empirical content than its predecessor need also be a hypothesis that is not independently testable of some explanandum of that predecessor.