What does thicken mean in cooking?

thicken means: A thickening agent is a substance that makes a liquid more dense. You can use gelatin, flour or heat to evaporate some liquid. (in Cooking Dictionary)

What else does thicken mean?

• A way to make thicker or more dense. (in Merlin Dictionary)
• Achieve greater intensity, complexity, or depth. (in Merlin Dictionary)