Source : Webster's Revised Unabridged Dictionary (1913)

Sharpen \Sharp"en\, v. t. [imp. & p. p. {Sarpened}; p. pr. & vb.
   n. {Sharpening}.] [See {Sharp}, a.]
   To make sharp. Specifically:
   (a) To give a keen edge or fine point to; to make sharper;
       as, to sharpen an ax, or the teeth of a saw.
   (b) To render more quick or acute in perception; to make more
       ready or ingenious.

             The air . . . sharpened his visual ray To objects
             distant far.                         --Milton.

             He that wrestles with us strengthens our nerves and
             sharpens our skill.                  --Burke.
   (c) To make more eager; as, to sharpen men's desires.

             Epicurean cooks Sharpen with cloyless sauce his
             appetite.                            --Shak.
   (d) To make more pungent and intense; as, to sharpen a pain
       or disease.
   (e) To make biting, sarcastic, or severe. ``Sharpen each
       word.'' --E. Smith.
   (f) To render more shrill or piercing.

             Inclosures not only preserve sound, but increase
             and sharpen it.                      --Bacon.
   (g) To make more tart or acid; to make sour; as, the rays of
       the sun sharpen vinegar.
   (h) (Mus.) To raise, as a sound, by means of a sharp; to
       apply a sharp to.

Sharpen \Sharp"en\, v. i.
   To grow or become sharp.

Source : WordNet®

sharpen
     v 1: make sharp or sharper; "sharpen the knives" [ant: {dull}]
     2: make sharp or sharper; "We had to sharpen our arguments"
     3: become sharp or sharper; "The debate sharpened"
     4: put (an image) into focus; "Please focus the image; we
        cannot enjoy the movie" [syn: {focus}, {focalize}, {focalise}]
        [ant: {blur}]
     5: make (images or sounds) sharp or sharper [ant: {soften}]
     6: raise the pitch of (musical notes) [ant: {flatten}]
     7: give a point to; "The candles are tapered" [syn: {taper}, {point}]
     8: make (one's senses) more acute; "This drug will sharpen your
        vision" [syn: {heighten}]