The Collaborative International Dictionary of English v.0.48:

Modus \Mo"dus\, n.; pl. Modi. [L. See Mode.] (Old Law)
   1. The arrangement of, or mode of expressing, the terms of a
      contract or conveyance.
      [1913 Webster]

   2. (Law) A qualification involving the idea of variation or
      departure from some general rule or form, in the way of
      either restriction or enlargement, according to the
      circumstances of the case, as in the will of a donor, an
      agreement between parties, and the like. --Bracton.
      [1913 Webster]

   3. (Law) A fixed compensation or equivalent given instead of
      payment of tithes in kind, expressed in full by the phrase
      modus decimandi. --Blackstone.
      [1913 Webster]

            They, from time immemorial, had paid a modus, or
            composition.                          --Landor.
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Bouvier's Law Dictionary, Revised 6th Ed (1856):

MODUS, civil law. Manlier; means; way.



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

MODUS, eccl. law. Where there is by custom a particular manner of tithing
allowed, different from the general law of taking tithes in kind, as a
pecuniary compensation, or the performance of labor, or when any means are
adopted by which the general law of tithing is altered, and a new method of
taking them is introduced, it is called a modus decimandi, or special manner
of taking tithes. 2 Bl. Com. 29.