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 ]