The Collaborative International Dictionary of English v.0.48:

Gyroidal \Gy*roid"al\ (j[-i]*roid"al), a. [Gr. gy^ros circle +
   -oid + -al.]
   1. Spiral in arrangement or action.
      [1913 Webster]

   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.
      [1913 Webster]

   3. (Opt.) Turning the plane of polarization circularly or
      spirally to the right or left.
      [1913 Webster]