The Collaborative International Dictionary of English v.0.48:

Advowson \Ad*vow"son\ (?; 277), n. [OE. avoweisoun, OF.
   avo["e]son, fr. L. advocatio. Cf. Advocation.] (Eng. Law)
   The right of presenting to a vacant benefice or living in the
   church. [Originally, the relation of a patron (advocatus) or
   protector of a benefice, and thus privileged to nominate or
   present to it.]
   [1913 Webster]

   Note: The benefices of the Church of England are in every
         case subjects of presentation. They are nearly 12,000
         in number; the advowson of more than half of them
         belongs to private persons, and of the remainder to the
         crown, bishops, deans and chapters, universities, and
         colleges. --Amer. Cyc.
         [1913 Webster]

WordNet (r) 3.0 (2006):

advowson
    n 1: the right in English law of presenting a nominee to a
         vacant ecclesiastical benefice