The Collaborative International Dictionary of English v.0.48:

Fiducial \Fi*du"cial\, a. [L. fiducia trust, confidence; akin to
   fides faith. See Faith.]
   1. Having faith or trust; confident; undoubting; firm.
      "Fiducial reliance on the promises of God." --Hammond.
      [1913 Webster]

   2. Having the nature of a trust; fiduciary; as, fiducial
      power. --Spelman.
      [1913 Webster]

   Fiducial edge (Astron. & Surv.), the straight edge of the
      alidade or ruler along which a straight line is to be
      drawn.

   Fiducial line or Fiducial point (Math. & Physics.), a
      line or point of reference, as for setting a graduated
      circle or scale used for measurements.
      [1913 Webster]