The Collaborative International Dictionary of English v.0.48:

Thickening \Thick"en*ing\, n.
   Something put into a liquid or mass to make it thicker.
   [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.]
       [1913 Webster]

             And this may to thicken other proofs. --Shak.
       [1913 Webster]
   (d) To make more frequent; as, to thicken blows.
       [1913 Webster]

WordNet (r) 3.0 (2006):

thickening
    adj 1: accumulating and becoming more intense; "the deepening
           gloom"; "felt a deepening love"; "the thickening dusk"
           [syn: deepening(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: thickening, thickener]
    2: any thickened enlargement [syn: node, knob, thickening]
    3: the act of thickening [syn: thickening, inspissation]

Moby Thesaurus II by Grady Ward, 1.0:

50 Moby Thesaurus words for "thickening":
   adhesiveness, clabbering, clamminess, clotting, coagulation,
   colloidality, concretion, congealment, congelation, curdling,
   doughiness, gelatination, gelatinity, gelatinization,
   gelatinousness, gelling, gluelikeness, gluiness, glutinosity,
   glutinousness, gumlikeness, gumminess, heaviness, incrassation,
   inspissation, jellification, jelling, jellying, jellylikeness,
   lentor, mucilaginousness, pastiness, ropiness, setting, slabbiness,
   sliminess, spissitude, stickiness, stodginess, stringiness,
   syrupiness, tackiness, tenaciousness, tenacity, thickness,
   toughness, treacliness, viscidity, viscosity, viscousness