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]

Moby Thesaurus II by Grady Ward, 1.0:

54 Moby Thesaurus words for "papa":
   Grand Penitentiary, Holy Father, abba, abuna, antipope, archbishop,
   archdeacon, archpriest, bishop, bishop coadjutor, canon, cardinal,
   cardinal bishop, cardinal deacon, cardinal priest, chaplain,
   coadjutor, curate, dad, daddy, dean, diocesan, ecclesiarch, exarch,
   father, foster father, genitor, governor, hierarch, high priest,
   metropolitan, old man, pa, pap, pappy, pater, paterfamilias,
   patriarch, penitentiary, pontiff, pop, pope, pops, prebendary,
   prelate, primate, rector, rural dean, sire, stepfather, subdean,
   suffragan, the old man, vicar