Make thick or thicker:
— Thicken the sauce.

synonym: inspissate.

Roget 321: be dense etc. adj.; become solid, render solid etc. adj.; solidify, solidate^†; concrete, set, take a set, consolidate, congeal, coagulate; curd, curdle; lopper; fix, clot, cake, candy, precipitate, deposit, cohere, crystallize; petrify etc. (harden) 323.    condense, thicken, gel, inspissate^†, incrassate^†; compress, squeeze, ram down, constipate.    ... show more

Roget 352: inspissate^†, incrassate^†; thicken, mash, squash, churn, beat up.    sinter.   

Roget 202: be broad etc. adj.; become broad, render broad etc. adj.; expand etc. 194; thicken, widen, calibrate.   

Dutch: binden

Become thick or thicker:
— The sauce thickened.

synonym: inspissate.

Dutch: verdikken

Make viscous or dense:
— Thicken the sauce by adding flour.

synonym: inspissate.