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]

Moby Thesaurus II by Grady Ward, 1.0:

35 Moby Thesaurus words for "Christianity":
   Christendom, Christianism, Christlikeness, Christliness,
   Judeo-Christian religion, Orthodox Christianity,
   Protestant Christianity, angelicalness, beauty of holiness,
   fear of God, godlikeness, godliness, godly-mindedness, goodness,
   heavenliness, heavenly-mindedness, holiness, holy-mindedness,
   odor of sanctity, otherworldliness, pureness, pureness of heart,
   purity, righteousness, sainthood, saintlikeness, saintliness,
   saintship, sanctitude, sanctity, seraphicalness,
   spiritual-mindedness, spirituality, unearthliness, unworldliness



Bouvier's Law Dictionary, Revised 6th Ed (1856):

CHRISTIANITY. The religion established by Jesus Christ.
     2. Christianity has been judicially declared to be a part of the common
law of Pennsylvania; 11 Serg. & Rawle, 394; 5 Binn. R.555; of New York, 8
Johns. R. 291; of Connecticut, 2 Swift's System, 321; of Massachusetts,
Dane's Ab. vol. 7, c. 219, a. 2, 19. To write or speak contemptuously and
maliciously against it, is an indictable offence. Vide Cooper on the Law of
Libel, 59 and 114, et seq.; and generally, 1 Russ. on Cr. 217; 1 Hawk, c. 5;
1 Vent. 293; 3 Keb. 607; 1 Barn. & Cress. 26. S. C. 8 Eng. Com. Law R. 14;
Barnard. 162; Fitzgib. 66; Roscoe, Cr. Ev. 524; 2 Str. 834; 3 Barn. & Ald.
161; S. C. 5 Eng. Com. Law R. 249 Jeff. Rep. Appx. See 1 Cro. Jac. 421 Vent.
293; 3 Keb. 607; Cooke on Def. 74; 2 How. S. C. 11 ep. 127, 197 to 201.