arcual measure of an angle

from The Collaborative International Dictionary of English v.0.48

Arcual \Arc"u*al\, a.
   Of or pertaining to an arc.
   [1913 Webster]

   {Arcual measure of an angle} (Math.), that in which the unit
      angle has its measuring arc equal to the radius of the
      circle.
      [1913 Webster] Arcuate