The term 'ad hoc' means something done or established for an immediate need (e.g., an ad hoc committee as opposed to a standing committee that is permanent.)
In research 'ad hoc' assumptions are those made to explain why observations do not match those anticipated on the basis of a hypothesis – they may sometimes be justified (they can be treated as hypotheses for further investigation), but can also be used to explain away undesirable results. It has therefore been suggested that "ad hoc explanations are very much like excuses" (Mario Bunge).
