From The Collaborative International Dictionary of English v.0.48:

Undulatory \Un"du*la*to*ry\ (?; 277), a. [Cf. F. ondulatoire.]
   Moving in the manner of undulations, or waves; resembling the
   motion of waves, which successively rise or swell rise or
   swell and fall; pertaining to a propagated alternating
   motion, similar to that of waves.
   [1913 Webster]

   Undulatory theory, or Wave theory (of light) (Opt.), that
      theory which regards the various phenomena of light as due
      to undulations in an ethereal medium, propagated from the
      radiant with immense, but measurable, velocities, and
      producing different impressions on the retina according to
      their amplitude and frequency, the sensation of brightness
      depending on the former, that of color on the latter. The
      undulations are supposed to take place, not in the
      direction of propagation, as in the air waves constituting
      sound, but transversely, and the various phenomena of
      refraction, polarization, interference, etc., are
      attributable to the different affections of these
      undulations in different circumstances of propagation. It
      is computed that the frequency of the undulations
      corresponding to the several colors of the spectrum ranges
      from 458 millions of millions per second for the extreme
      red ray, to 727 millions of millions for the extreme
      violet, and their lengths for the same colors, from the
      thirty-eight thousandth to the sixty thousandth part of an
      inch. The theory of ethereal undulations is applicable not
      only to the phenomena of light, but also to those of heat.
      [1913 Webster]
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