Impacted wisdom tooth (or teeth) is a dental condition whereby the third molar is prevented from erupting into the mouth fully or at all.  This is caused by a physical barrier such as other teeth in the way, by reduced space due to a small mouth and big teeth, and by poor angulation during eruption.  These conditions can result in a partially erupted wisdom tooth.
Complete Impaction is when the wisdom tooth (or teeth) is present in the jaw but unerupted in the mouth.  Indication of this is confirmed by a physical exam and an x-ray such as a Panorex.