The Collaborative International Dictionary of English v.0.48:

Optically \Op"tic*al*ly\, adv.
   1. By optics or sight.
      [1913 Webster]

   2. With reference to optics.
      [1913 Webster]

   Optically active, Optically inactive (Chem. Physics),
      terms used of certain isomeric substances which, while
      identical with each other in other respects, differ in
      this, viz., that they do or do not produce right-handed or
      left-handed circular polarization of light. See optical
      activity.

   Optically positive, Optically negative. See under
      Refraction.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

optical activity \optical activity\ n. (Chem.)
   the ability of a chemical compound to rotate the plane of
   plane-polarized light. This is found only in chemicals whose
   molecular structure does not have a plane of symmetry, and is
   common in biochemical compounds. -- optically active adj.
   [PJC]