Culinary definition of coat: A traditional 'doneness' test which refers to a thickened liquids ability to 'coat' the back of a spoon and leave a thin film.
C@H FOOD FOR THOUGHT
We refer to coating the back of a spoon, most-often in reference to reduction sauces made with pan drippings, and fruit sauces/glazes for desserts and ice cream. Most other sauces, cheese sauce included, are too thick to coat the back of a spoon.