The Collaborative International Dictionary of English v.0.48:

Peanut gallery \Pea"nut gal"ler*y\
   1. The cheapest seats in a theater, being typically those
      farthest from the stage, as in the uppermost balcony.
      [PJC]

   2. In a television show for children, the section where
      visiting children sit as guests during the performance.
      [PJC]

   3. Hence: Any group of children, or the chidren in a group of
      people; as, let's have no noise from the peanut gallery.
      [Colloq.]
      [PJC]

WordNet (r) 3.0 (2006):

peanut gallery
    n 1: (figurative) people whose criticisms are regarded as
         irrelevant or insignificant (resembling uneducated people
         who throw peanuts on the stage to express displeasure with
         a performance); "he ignored complaints from the peanut
         gallery"
    2: rearmost or uppermost area in the balcony containing the
       least expensive seats [syn: second balcony, family
       circle, upper balcony, peanut gallery]