A thickener is a substance added to liquids to make them thicker, more viscous, or less watery. When you make gravy from thin pan drippings, you might whisk in flour or cornstarch as a thickener to give it that rich, coating consistency. Ice cream manufacturers use thickeners to create a smooth, creamy texture instead of an icy one.

Thickeners work by absorbing water or creating networks of molecules that trap liquid, changing how it flows. Cornstarch thickens pie filling, gelatin thickens pudding, and pectin thickens jam. Even salad dressings use thickeners to keep oil and vinegar from separating immediately.

The word can also describe something that makes situations more complicated or intense. If a mystery novel introduces a surprising witness, you might say the plot thickens, meaning the story just got more complex and interesting. But the culinary meaning is far more common: when a recipe tells you to add a thickener, it wants you to transform a thin, runny liquid into something with more body and substance.