Thicken/ˈθɪkən/ Definition 1 of 3Pro Tip 1/3Cooking Context Used often in cooking to describe the process of making a liquid ingredient denser. Keep stirring as you thicken the sauce. Pro Tip 2/3Physical Change Implies a change in texture that can be seen or felt. The fog will thicken overnight. Pro Tip 3/3Gradual Process The change generally happens gradually, not instantly. Wait for the stew to thicken before serving.