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.
      [1913 Webster]

   2.
      (a) (Zool.) The spongy interior substance of a feather.
      (b) (Anat.) The spinal cord; the marrow.
          [1913 Webster]

   3. Hence: The which contains the strength of life; the vital
      or essential part; concentrated force; vigor; strength;
      importance; as, the speech lacked pith.
      [1913 Webster]

            Enterprises of great pith and moment. --Shak.
      [1913 Webster]

   Pith paper. Same as Rice paper, under Rice.
      [1913 Webster]

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)

Moby Thesaurus II by Grady Ward, 1.0:

204 Moby Thesaurus words for "pith":
   affective meaning, axiom, axis, backbone, bearing, bench mark,
   bottom, burden, butter, cardinal point, cataplasm, center,
   center of action, center of gravity, centroid, centrum,
   chief thing, chutzpah, climax, coloring, connotation, consequence,
   core, cornerstone, courage, crisis, critical point, crush, crux,
   dead center, denotation, dental pulp, depth, distillate,
   distillation, drift, effect, elixir, epicenter, essence, essential,
   essential matter, essentiality, extension, fabric, flower,
   focal point, focus, force, fulcrum, fundamental, gameness, gist,
   grammatical meaning, gravamen, gravity, great point, grit, guts,
   gutsiness, guttiness, heart, heart of oak, high point, hub,
   hypostasis, idea, impact, implication, import, importance,
   important thing, inner essence, intension, intestinal fortitude,
   issue, kernel, keystone, landmark, lexical meaning,
   literal meaning, magnitude, main point, main thing, marrow, mash,
   material, material point, matter, meaning, meaningfulness, meat,
   medium, medulla, metacenter, mettle, mettlesomeness, middle,
   milestone, moment, momentousness, moxie, mush, nave, navel, nerve,
   nub, nucleus, nuts and bolts, omphalos, overtone, paper pulp,
   paste, pertinence, pivot, plaster, pluck, pluckiness, point,
   porridge, postulate, poultice, practical consequence, principle,
   pudding, pulp, pulp lead, pulpwood, purport, quick, quid, quiddity,
   quintessence, quintessential, rag pulp, range of meaning,
   real issue, real meaning, recap, recapitulation, reference,
   referent, relation, relevance, resume, rock bottom, root,
   run-through, rundown, salient point, sap, sauce, scope,
   semantic cluster, semantic field, sense, short, significance,
   signification, significatum, signifie, sine qua non, smash, soul,
   span of meaning, spirit, sponge, spunk, spunkiness, squash,
   stamina, storm center, stout heart, structural meaning, stuff,
   substance, substantive point, sulfate pulp, sulfite pulp, sum,
   sum and substance, summary, summation, symbolic meaning, tenor,
   the bottom line, the nitty-gritty, the point, thrust,
   totality of associations, toughness, transferred meaning,
   true grit, turning point, umbilicus, unadorned meaning, undertone,
   upshot, value, virtuality, weight, weightiness, white lead,
   wood pulp