Hygroscopic describes a substance that naturally attracts and absorbs water from the surrounding air. Sometimes this word gets confused with "hydrophillic," which just means anything that has an affinity for water. While hygroscopic ingredients are also hydrophilic, being hygroscopic specifically means they actively pull moisture from the air and hold onto it.

In baking, the hygroscopic ingredients we usually refer to are sugars (either in solid-granulated or syrup form), with each type (or molecule) varying in how strongly they attract water. In general, fructose-rich sweeteners such as honey, agave nectar, and invert sugar are among the most hygroscopic, followed by corn syrup, brown sugar (because of its molasses), glucose syrup, and finally granulated sugar.