You call something ad hoc when it's introduced for a particular purpose, instead of for some general, antecedently motivated reason. So, for instance, an ad hoc decision is a decision you make when there's no general rule or precedent telling you what to do.

Philosophers sometimes accuse their opponents of making ad hoc hypotheses (or ad hoc stipulations, or ad hoc amendments to their analyses, etc.). These are hypotheses (or stipulations or amendments) adopted purely for the purpose of saving a theory from difficulty or refutation, without any independent motivation or rationale. They will usually strike the reader as artificial or "cheating."

For instance, suppose you analyze "bird" as "any creature that can fly." I then cite mosquitos as a counter-example. They can fly, but they aren't birds. Now, you might fix up your analysis as follows:

A bird is any creature that can fly, and which is not a mosquito.

This would be an ad hoc response to my counter-example. Alternatively, you might fix up your analysis as follows:

A bird is any creature that can fly, and which has a backbone.

This would be an independently motivated, and more appropriate, response to my counter-example. (Of course, someone may discover counter-examples even to this revised analysis.)