inspissate

(verb) make viscous or dense. Synonyms: thicken. Example: "Thicken the sauce by adding flour."

(verb) make thick or thicker. Synonyms: thicken. Examples: "Thicken the sauce." "Inspissate the tar so that it becomes pitch."

(verb) become thick or thicker. Synonyms: thicken. Examples: "The sauce thickened." "The egg yolk will inspissate."