The Collaborative International Dictionary of English v.0.48:

Papa \Pa*pa"\, n. [F. papa, L. papa; cf. Gr. ?, ?, a child's
   word meaning father. Cf. Pope.]
   [1913 Webster]
   1. A child's word for father.
      [1913 Webster]

   2. A parish priest in the Greek Church. --Shipley.
      [1913 Webster]

WordNet (r) 3.0 (2006):

papa
    n 1: an informal term for a father; probably derived from baby
         talk [syn: dad, dada, daddy, pa, papa, pappa,
         pop]