The Collaborative International Dictionary of English v.0.48:

Purana \Pu*ra"na\, n. [Skr. pur[=a]?, properly. old, ancient,
   fr. pur[=a] formerly.]
   One of a class of sacred Hindoo poetical works in the
   Sanskrit language which treat of the creation, destruction,
   and renovation of worlds, the genealogy and achievements of
   gods and heroes, the reigns of the Manus, and the
   transactions of their descendants. The principal Puranas are
   eighteen in number, and there are the same number of
   supplementary books called Upa Puranas.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Purana
    n 1: a body of 18 works written between the first and 11th
         centuries and incorporating legends and speculative
         histories of the universe and myths and customary
         observances