Source : Webster's Revised Unabridged Dictionary (1913)

Gyroidal \Gy*roid"al\, a. [Gr. ? circle + -oid + -al.]
   1. Spiral in arrangement or action.

   2. (Crystallog.) Having the planes arranged spirally, so that
      they incline all to the right (or left) of a vertical
      line; -- said of certain hemihedral forms.

   3. (Opt.) Turning the plane of polarization circularly or
      spirally to the right or left.