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.

Thickening \Thick"en*ing\, n.
   Something put into a liquid or mass to make it thicker.

Source : WordNet®

thickening
     adj 1: accumulating and becoming more intense; "the deepening
            gloom"; "felt a deepening love"; "the gathering
            darkness"; "the thickening dusk" [syn: {deepening(a)},
             {gathering(a)}, {thickening(a)}]
     2: becoming more intricate or complex; "a thickening plot"
     n 1: any material used to thicken; "starch is used in cooking as
          a thickening" [syn: {thickener}]
     2: any thickened enlargement [syn: {node}, {knob}]
     3: the act of thickening [syn: {inspissation}]