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.
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   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.
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   3. (Law) A fixed compensation or equivalent given instead of
      payment of tithes in kind, expressed in full by the phrase
      modus decimandi. --Blackstone.
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            They, from time immemorial, had paid a modus, or
            composition.                          --Landor.
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