The Collaborative International Dictionary of English v.0.48:

Thicken \Thick"en\, v. i.
   To become thick. "Thy luster thickens when he shines by."
   --Shak.
   [1913 Webster]

         The press of people thickens to the court. --Dryden.
   [1913 Webster]

         The combat thickens, like the storm that flies.
                                                  --Dryden.
   [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Thicken \Thick"en\, v. t. [imp. & p. p. Thickened; p. pr. &
   vb. n. Thickening.]
   To make thick (in any sense of the word). Specifically:
   [1913 Webster]
   (a) To render dense; to inspissate; as, to thicken paint.
       [1913 Webster]
   (b) To make close; to fill up interstices in; as, to thicken
       cloth; to thicken ranks of trees or men.
       [1913 Webster]
   (c) To strengthen; to confirm. [Obs.]
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             And this may to thicken other proofs. --Shak.
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   (d) To make more frequent; as, to thicken blows.
       [1913 Webster]

WordNet (r) 3.0 (2006):

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