A gratin is any dish that is topped with cheese or breadcrumbs mixed with bits of butter, then heated in the oven or under the broiler until brown and crispy. The terms au gratin or gratinée refer to any dish prepared in such a manner. Special round or oval gratin pans and dishes are ovenproof and shallow, which increases a dish's surface area, thereby insuring a larger crispy portion for each serving.
From The Food Lover's Companion, Fourth edition by Sharon Tyler Herbst and Ron Herbst. Copyright © 2007, 2001, 1995, 1990 by Barron's Educational Series, Inc.