The Collaborative International Dictionary of English v.0.48:

Sunna \Sun"na\, n. [Ar. sunnah rule, law.]
   A collection of traditions received by the orthodox
   Mohammedans as of equal authority with the Koran.
   [1913 Webster]

WordNet (r) 3.0 (2006):

Sunna
    n 1: (Islam) the way of life prescribed as normative for Muslims
         on the basis of the teachings and practices of Muhammad and
         interpretations of the Koran [syn: Sunnah, Sunna,
         hadith]