Source : Webster's Revised Unabridged Dictionary (1913)

Prehistoric \Pre`his*tor"ic\, a.
   Of or pertaining to a period before written history begins;
   as, the prehistoric ages; prehistoric man.

Source : WordNet®

prehistoric
     adj 1: belonging to or existing in times before recorded history;
            "prehistoric settlements"; "prehistoric peoples" [syn:
             {prehistorical}]
     2: of or relating to times before written history; "prehistoric
        archeology"
     3: no longer fashionable; "my mother has these prehistoric
        ideas about proper clothes"