The Collaborative International Dictionary of English v.0.48:

Substraction \Sub*strac"tion\, n. [OF. substraction, F.
   soustraction. See Subtract.]
   1. Subtraction; deduction. [Obs.]
      [1913 Webster]

   2. (Law) See Subtraction, 3.
      [1913 Webster]

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

SUBSTRACTION, French law. The act of taking something fraudulently; it is
generally applied to the taking of the goods of the estate of a deceased
person fraudulently. Vide Expilation.