The word entrée in French originally denoted the "entry" of the dishes from the kitchens into the dining hall. In the illustration from a French fifteenth-century illuminated manuscript of the Histoire d'Olivier de Castille et d'Artus d'Algarbe, a fanfare from trumpeters in the musicians' gallery announces the processional entrée of a series of dishes....For us as for 15th century diners the entree is the showily-presented main course, which in terms of Thanksgiving would be when the Turkey is brought to the table on a big platter and carved for everyone.
In traditional French haute cuisine, the entrée preceded a larger dish known as the relevé, which "replaces" or "relieves" it, an obsolete term in modern cooking, but still used as late as 1921 in Escoffier's Le Guide Culinaire.
In France, the modern restaurant menu meaning of "entrée" is the course that precedes the main course in a three-course meal, i.e. the course which in British usage is often called the "starter" and in American usage the "appetizer."
More on Medieval Thanksgiving
While looking up something that I was thinking about with regard to Eric's comment on the Ancient Roman use of spices, I learned something that I did not know: the way that we use the term "entree" in North American English is not just different from the way the French use it, it's different from the way everyone else in the world uses it. But it is not different because it's an American innovation. It's different because we retain the Medieval meaning of the word.
By Grim on Friday, November 27, 2015