Thicken

Culinary definition of thicken:  To increase the thickness (viscosity) of liquids by heat reduction (water evaporation), or by adding thickening agents like flour, cornstarch, and eggs. 

C@H FOOD FOR THOUGHT
If you have the time, we recommend thickening sauces and gravies by first cooking down to reduce water content, then adding flour or cornstarch only if needed.  Reduction concentrates colour and flavours while thickening agents can sometimes add a floury or starchy taste, and cause cloudiness.  

Never add flour or cornstarch directly to the base sauce or gravy as lumps will always form (if this happens, run sauce through a sieve to remove as many lumps as possible), rather in a separate cup, make a smooth loose paste of a teaspoon or two of flour or starch mixed with a small amount of water and add this mixture to the gravy slowly - stirring constantly.  The base liquid will thicken quickly, but only as the starches reach temperature.  

Whisked or beaten eggs and egg yolks are used to thicken custards and desserts like vanilla creme Anglaise (above); be sure to temper eggs slowly with the small amounts of the hot base, before adding the eggs in their entirety to the base. Otherwise, the egg mixture clumps and cooks separately from the base instead of incorporating into it.  To ensure silky smooth consistency, egg-thickened custards and cremes should be passed through a fine sieve - to remove small lumps - before serving.  See also Roux.

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