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

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

ADVOWSON, ecclesiastical law. From advow or advocare, a right of
presentation to a church or benefice. He who possesses this right is called
the patron or advocate, (q.v.) when there is no patron, or he neglects to
exercise his right within six months, it is called a lapse, i. e. a title is
given to the ordinary to collate to a church; when a presentation is made by
one who has no right it is called a usurpation.
     2. Advowsons are of different kinds, as Advowson appendant, when it
depends upon a manor, &c. - Advowson in gross, when it belongs to a person
and not to a manor. - Advowson presentative, where the patron presents to
the bishop. - Advowson donative, where the king or patron puts the clerk
into possession without presentation. - Advowson of the moiety of the
church, where there are two several patrons and two incumbents in the same
church. - A moiety of advowson, where two must join the presentation, of one
incumbent. - Advowson of religious houses, that which is vested in the
person who founded such a house. Techn. Dict.; 2 Bl. Com. 21; Mirehouse on
Advowsons; Com. Dig. Advowson, Quare Impedit; Bac. Ab. Simony; Burn's Eccl.
Law, h.t.; Cruise's Dig. Index, h.t.