The Collaborative International Dictionary of English v.0.48:

Impropriation \Im*pro`pri*a"tion\, n.
   1. The act of impropriating; as, the impropriation of
      property or tithes; also, that which is impropriated.
      [1913 Webster]

   2. (Eng. Eccl. Law)
      (a) The act of putting an ecclesiastical benefice in the
          hands of a layman, or lay corporation.
      (b) A benefice in the hands of a layman, or of a lay
          corporation.
          [1913 Webster]

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

IMPROPRIATION, eccl. law. The act, of employing the revenues of a church
living to one's own use; it is also a parsonage or ecclesiastical living in
the hands of a layman, or which descends by inheritance. Tech. Dict. h.t.