The Collaborative International Dictionary of English v.0.48:

Natant \Na"tant\, a. [L. natans, -antis, from swim, v. intens.
   fr. nare to swim: cf. F. natant.]
   1. (Bot.) Floating in water, as the leaves of water lilies,
      or submersed, as those of many aquatic plants.
      [1913 Webster]

   2. (Her.) Placed horizontally across the field, as if
      swimming toward the dexter side; said of all sorts of
      fishes except the flying fish.
      [1913 Webster]