Source : Webster's Revised Unabridged Dictionary (1913)

Thicken \Thick"en\, v. t. [imp. & p. p. {Thickened}; p. pr. &
   vb. n. {Thickening}.]
   To make thick (in any sense of the word). Specifically:
   (a) To render dense; to inspissate; as, to thicken paint.
   (b) To make close; to fill up interstices in; as, to thicken
       cloth; to thicken ranks of trees or men.
   (c) To strengthen; to confirm. [Obs.]

             And this may to thicken other proofs. --Shak.
   (d) To make more frequent; as, to thicken blows.

Thicken \Thick"en\, v. i.
   To become thick. ``Thy luster thickens when he shines by.''
   --Shak.

         The press of people thickens to the court. --Dryden.

         The combat thickens, like the storm that flies.
                                                  --Dryden.

Source : WordNet®

thicken
     v 1: make thick or thicker; "Thicken the sauce"; "inspissate the
          tar so that it becomes pitch" [syn: {inspissate}] [ant:
          {thin}]
     2: become thick or thicker; "The sauce thickened"; "The egg
        yolk will inspissate" [syn: {inspissate}] [ant: {thin}]
     3: make viscous or dense; "thicken the sauce by adding flour"
        [syn: {inspissate}]