The Collaborative International Dictionary of English v.0.48:

Epigram \Ep"i*gram\, n. [L. epigramma, fr. Gr. ? inscription,
   epigram, fr. ? to write upon, 'epi` upon + ? to write: cf. F.
   ['e]pigramme. See Graphic.]
   1. A short poem treating concisely and pointedly of a single
      thought or event. The modern epigram is so contrived as to
      surprise the reader with a witticism or ingenious turn of
      thought, and is often satirical in character.
      [1913 Webster]

            Dost thou think I care for a satire or an epigram?
                                                  --Shak.
      [1913 Webster]

   Note: Epigrams were originally inscription on tombs, statues,
         temples, triumphal arches, etc.
         [1913 Webster]

   2. An effusion of wit; a bright thought tersely and sharply
      expressed, whether in verse or prose.
      [1913 Webster]

   3. The style of the epigram.
      [1913 Webster]

            Antithesis, i. e., bilateral stroke, is the soul of
            epigram in its later and technical signification.
                                                  --B. Cracroft.
      Epigrammatic

WordNet (r) 3.0 (2006):

epigram
    n 1: a witty saying [syn: epigram, quip]

The Devil's Dictionary (1881-1906):

EPIGRAM, n.  A short, sharp saying in prose or verse, frequently
characterize by acidity or acerbity and sometimes by wisdom.
Following are some of the more notable epigrams of the learned and
ingenious Dr. Jamrach Holobom:

        We know better the needs of ourselves than of others.  To
    serve oneself is economy of administration.

        In each human heart are a tiger, a pig, an ass and a
    nightingale.  Diversity of character is due to their unequal
    activity.

        There are three sexes; males, females and girls.

        Beauty in women and distinction in men are alike in this:
    they seem to be the unthinking a kind of credibility.

        Women in love are less ashamed than men.  They have less to be
    ashamed of.

        While your friend holds you affectionately by both your hands
    you are safe, for you can watch both his.