Source : Webster's Revised Unabridged Dictionary (1913)

Petitioner \Pe*ti"tion*er\, n.
   One who presents a petition.

Source : WordNet®

petitioner
     n 1: one praying humbly for something; "a suppliant for her
          favors" [syn: {suppliant}, {supplicant}]
     2: someone who petitions a court for redress of a grievance or
        recovery of a right [syn: {suer}]