The Collaborative International Dictionary of English v.0.48:

Plexus \Plex"us\, n.; pl. L. Plexus, E. Plexuses. [L., a
   twining, braid, fr. plectere, plexum, to twine, braid.]
   1. (Anat.) A network of vessels, nerves, or fibers.
      [1913 Webster]

   2. (Math.) The system of equations required for the complete
      expression of the relations which exist between a set of
      quantities. --Brande & C.
      [1913 Webster]

   3. A network; an intricate or interwoven combination of
      elements or parts in a coherent structure.

            In the perception of a tree the reference to an
            object is circumscribed and directed by a plexus of
            visual and other presentations.       --G. F. Stout.
      [Webster 1913 Suppl.]