The word entrée (pronounced "ON-tray") in the culinary arts can be confusing because it's one of those words that means the precise opposite in some parts of the world of what it means in other parts.
In the United States, the word entrée is often used to signify the part of a meal that you would think of as the main course.
In other places, however, it is not uncommon to hear the word entrée used to indicate an appetizer, first course or starter.
For the most part, this confusion isn't too serious, unless you happen to be ordering in a restaurant in a country where the word entrée means the opposite of what you think it means. In which case, you might end up getting your starter as your main course or vice versa.
Take the following sentence, for example:
"For my entrée, I'll have the soup."
In the U.S., saying that sentence will get you the soup as your main course. In most of the rest of the world, you'll get the soup as your appetizer.
If you're not sure, you could always specify "main course" or "appetizer," instead of entrée, just to be clear. Or "first course" and "second course."
Speaking of clarity, allow me now to introduce a little bit less. Because it so happens that there is yet another sense of the word entrée, which can refer specifically to the protein (or otherwise central) component of a dish, as opposed to its accompaniments.
Certain steakhouses, for example, allow a patron to select from various steak offerings, such as a ribeye, New York strip or Porterhouse, and whatever steak they choose is the entrée. Diners select their accompaniments or side dishes, such as a baked potato or choice of vegetable, separately.
Thus the steak on its own is the entrée, and the potato and vegetable are the sides.
Further Confusion on Definitions
How, though, did the usage of a word come to include what it actually means as well as the opposite of its meaning? (The word "literally" appears to be a recent example of this.)
After all, the word entrée in French means beginning.
The non-explanation stems from the way traditional French dinners were served. The first course would usually be a soup, followed by an intermediate course usually consisting of fish.
After the fish course would come the entrée, which might feature poultry, or lobster, or possibly even a cold item such as aspic, chaud-froid or pâté. A meat course, such as roast beef or lamb, would then follow the entrée. Vegetables, side dishes and sweet items would follow.
Thus the word entrée, which in French means beginning, referred to the third course of a classical menu. In other words, if you're confused, it's not your fault.