The Collaborative International Dictionary of English v.0.48:

Sectarianism \Sec*ta"ri*an*ism\, n.
   The quality or character of a sectarian; devotion to the
   interests of a party; excess of partisan or denominational
   zeal; adherence to a separate church organization.
   [1913 Webster]

WordNet (r) 3.0 (2006):

sectarianism
    n 1: a narrow-minded adherence to a particular sect or party or
         denomination; "he condemned religious sectarianism" [syn:
         sectarianism, denominationalism]