thickening

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.

[…] the inner layer (of nearly equally large cells) lacks the regular semiannular or annular thickenings of most other leafy liverworts […]

They disappear at once on slightly relaxing the air-pressure, whilst true incipient thickenings of the surface remain white and unpliably stiff.

Beginning to thicken, becoming thicker.

From Keighley onwards we had obviously returned to civilisation, for the surrounding country was now studded with the sodium street lights of suburbia and a thickening industrial haze was blotting out the moon.