The Collaborative International Dictionary of English v.0.48:

Christianity \Chris*tian"i*ty\, n. [OE. cristiente, OF.
   cristient['e], F. chr['e]tient['e], fr. L. christianitas. ]
   [1913 Webster]
   1. The religion of Christians; the system of doctrines and
      precepts taught by Christ.
      [1913 Webster]

   2. Practical conformity of one's inward and outward life to
      the spirit of the Christian religion
      [1913 Webster]

   3. The body of Christian believers. [Obs.]
      [1913 Webster]

            To Walys fled the christianitee
            Of olde Britons.                      --Chaucer.
      [1913 Webster]

WordNet (r) 3.0 (2006):

Christianity
    n 1: a monotheistic system of beliefs and practices based on the
         Old Testament and the teachings of Jesus as embodied in the
         New Testament and emphasizing the role of Jesus as savior
         [syn: Christianity, Christian religion]
    2: the collective body of Christians throughout the world and
       history (found predominantly in Europe and the Americas and
       Australia); "for a thousand years the Roman Catholic Church
       was the principal church of Christendom" [syn: Christendom,
       Christianity]