What does the term "au gratin" mean when it comes to cooking?

What does the term “au gratin” mean when it comes to cooking? It means topped with a browned crust. The crust forms when the dish is baked or placed under a broiler or overhead grill.

Breadcrumbs are one common topping, but they are not required. Cheese, egg, or combinations of these ingredients can also create the characteristic golden surface. Therefore, “covered with browned breadcrumbs” is too narrow to be the canonical definition.

The term is associated with the French culinary technique called gratin. It can be applied to potatoes, vegetables, pasta, meat, fish, and other foods prepared in a shallow dish. The finished dish is often served in the same baking vessel in which its top was browned.