The Collaborative International Dictionary of English v.0.48:

Substance \Sub"stance\, n. [F., fr. L. substantia, fr. substare
   to be under or present, to stand firm; sub under + stare to
   stand. See Stand.]
   1. That which underlies all outward manifestations;
      substratum; the permanent subject or cause of phenomena,
      whether material or spiritual; that in which properties
      inhere; that which is real, in distinction from that which
      is apparent; the abiding part of any existence, in
      distinction from any accident; that which constitutes
      anything what it is; real or existing essence.
      [1913 Webster]

            These cooks, how they stamp, and strain, and grind,
            And turn substance into accident!     --Chaucer.
      [1913 Webster]

            Heroic virtue did his actions guide,
            And he the substance, not the appearance, chose.
                                                  --Dryden.
      [1913 Webster]

   2. The most important element in any existence; the
      characteristic and essential components of anything; the
      main part; essential import; purport.
      [1913 Webster]

            This edition is the same in substance with the
            Latin.                                --Bp. Burnet.
      [1913 Webster]

            It is insolent in words, in manner; but in substance
            it is not only insulting, but alarming. --Burke.
      [1913 Webster]

   3. Body; matter; material of which a thing is made; hence,
      substantiality; solidity; firmness; as, the substance of
      which a garment is made; some textile fabrics have little
      substance.
      [1913 Webster]

   4. Material possessions; estate; property; resources.
      [1913 Webster]

            And there wasted his substance with riotous living.
                                                  --Luke xv. 13.
      [1913 Webster]

            Thy substance, valued at the highest rate,
            Can not amount unto a hundred marks.  --Shak.
      [1913 Webster]

            We are destroying many thousand lives, and
            exhausting our substance, but not for our own
            interest.                             --Swift.
      [1913 Webster]

   5. (Theol.) Same as Hypostasis, 2.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Substance \Sub"stance\, v. t.
   To furnish or endow with substance; to supply property to; to
   make rich. [Obs.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

substance
    n 1: the real physical matter of which a person or thing
         consists; "DNA is the substance of our genes"
    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]
    3: the idea that is intended; "What is the meaning of this
       proverb?" [syn: meaning, substance]
    4: material of a particular kind or constitution; "the immune
       response recognizes invading substances"
    5: considerable capital (wealth or income); "he is a man of
       means" [syn: means, substance]
    6: what a communication that is about something is about [syn:
       message, content, subject matter, substance]
    7: a particular kind or species of matter with uniform
       properties; "shigella is one of the most toxic substances
       known to man"