Definify.com

Webster 1913 Edition

Noun.

Something put into a liquid or mass to make it thicker.

ppr.

Making dense or more dense, more close, or more frequent; inspissating.

Noun.

Something put into a liquid or mass to make it more thick.

thickening ‎(plural thickenings)

The process of making something, or becoming, thick or viscous.
A substance, usually a source of starch, used to thicken a sauce.
A thickened part of a structure.
- 1992, Rudolf M. Schuster, The Hepaticae and Anthocerotae of North America: East of the Hundredth Meridian, volume V, page 6
  […] the inner layer (of nearly equally large cells) lacks the regular semiannular or annular thickenings of most other leafy liverworts […]
- 1902, Edwin Hurry Fenwick, Obscure Diseases of the Urethra (page 30)
  They disappear at once on slightly relaxing the air-pressure, whilst true incipient thickenings of the surface remain white and unpliably stiff.

the process of making something, or becoming, thick or viscous

a substance, usually a source of starch, used to thicken a sauce

thickening