The Collaborative International Dictionary of English v.0.48:

paparazzo \paparazzo\ pl. paparazzi. n. [from Signore
   Paparazzo, a photographer in Fellini's film La Dolce Vita.]
   1. A free-lance photographer that specializes in following
      and photographing celebrities such as movie stars,
      especially to obtain candid photographs in private
      situations; as, her dogged pursuit by the paparazzi was
      believed to be a major factor in Princess Diana's death.
      [PJC]

WordNet (r) 3.0 (2006):

paparazzo
    n 1: a freelance photographer who pursues celebrities trying to
         take candid photographs of them to sell to newspapers or
         magazines