The Collaborative International Dictionary of English v.0.48:

Shiite \Shi"ite\, Shiah \Shi"ah\, n. [Ar. sh[imac]'a[imac]a
   follower of the sect of Ali, fr. sh[imac]'at, sh[imac]'ah, a
   multitude following one another in pursuit of the same
   object, the sect of Ali, fr. sh[=a]'a to follow.]
   A member of that branch of the Mohammedans to which the
   Persians belong. They reject the first three caliphs, and
   consider Ali as being the first and only rightful successor
   of Mohammed. They do not acknowledge the Sunna, or body of
   traditions respecting Mohammed, as any part of the law, and
   on these accounts are treated as heretics by the Sunnites, or
   orthodox Mohammedans.
   [1913 Webster] Shikaree

WordNet (r) 3.0 (2006):

Shiite
    n 1: a member of the branch of Islam that regards Ali as the
         legitimate successor to Mohammed and rejects the first
         three caliphs [syn: Shiite, Shi'ite, Shiite Muslim,
         Shi'ite Muslim, Shia Muslim]