1. [adv]  for one specific case; "they were appointed ad hoc"
2. [adj]  for or concerned with one specific purpose; "a coordinated policy instead of ad hoc decisions"
3. [adj]  often improvised or impromptu; "an ad hoc committee meeting"

Contrived purely for the purpose in hand rather than planned carefully in advance. E.g. "We didn't know what to do about the sausage rolls, so we set up an ad-hoc committee".