Source : Webster's Revised Unabridged Dictionary (1913)

Presupposition \Pre*sup`po*si"tion\, n. [Pref. pre- +
   supposition: cf. F. pr['e]supposition.]
   1. The act of presupposing; an antecedent implication;
      presumption.

   2. That which is presupposed; a previous supposition or
      surmise.

Source : WordNet®

presupposition
     n : the act of presupposing; a supposition made prior to having
         knowledge (as for the purpose of argument)