The Collaborative International Dictionary of English v.0.48:

Pith \Pith\, n. [AS. pi?a; akin to D. pit pith, kernel, LG.
   peddik. Cf. Pit a kernel.]
   1. (Bot.) The soft spongy substance in the center of the
      stems of many plants and trees, especially those of the
      dicotyledonous or exogenous classes. It consists of
      cellular tissue.
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   2.
      (a) (Zool.) The spongy interior substance of a feather.
      (b) (Anat.) The spinal cord; the marrow.
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   3. Hence: The which contains the strength of life; the vital
      or essential part; concentrated force; vigor; strength;
      importance; as, the speech lacked pith.
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            Enterprises of great pith and moment. --Shak.
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   Pith paper. Same as Rice paper, under Rice.
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The Collaborative International Dictionary of English v.0.48:

Pith \Pith\, v. t. (Physiol.)
   To destroy the central nervous system of (an animal, as a
   frog), as by passing a stout wire or needle up and down the
   vertebral canal.
   [1913 Webster]

WordNet (r) 3.0 (2006):

pith
    n 1: soft spongelike central cylinder of the stems of most
         flowering plants
    2: the choicest or most essential or most vital part of some
       idea or experience; "the gist of the prosecutor's argument";
       "the heart and soul of the Republican Party"; "the nub of the
       story" [syn: kernel, substance, core, center,
       centre, essence, gist, heart, heart and soul,
       inwardness, marrow, meat, nub, pith, sum, nitty-
       gritty]
    v 1: remove the pith from (a plant)