Crystalline means having molecules arranged in an organized, grid-like or repeating structure. You can think of it as saying something "has order." This description stems from cystals (you know, the rocks) because they have very distinct, ordered formations within that make up their final structure.

In baking we often think about fats, where crystallization happens when some of the fat molecules organize into these ordered structures and become solid. Milkfat, cocoa butter, and many other fats can crystallize. This is an important distinction because the way a fat crystallizes affects properties such as when it melts or sets, how firm or soft it feels at different temperatures, and how it performs in foods.

Other ingredients also have some neat crystallized structures - think white granulated sugar (sucrose), salt (sodium chloride), or even ice (ordered water molecules)!