The noun coronation chicken (also with capital initial(s)) denotes a dish of cold cooked chicken, usually diced, served in a mild creamy curry sauce.

This dish is so named because it was originally created for a lunch held to mark the coronation of Queen Elizabeth II, on Tuesday 2^nd June 1953, at which it was called Poulet Reine Elizabeth.

The noun coronation chicken was apparently coined by the British educator, florist and author Constance Spry (née Fletcher – 1886-1960) and the British cook and author Rosemary Hume (1907-1984) in The Constance Spry Cookery Book (London: J. M. Dent & Sons Ltd, 1956); this is the relevant passage from this book:
—from the reprinted edition (London: George Weidenfeld & Nicolson Ltd, 1994):

[page 1030]
The most exciting party was one given on Coronation Day, for which preparations were shared by the students and staff of the Cordon Bleu School in London and of Winkfield. This was a luncheon for the representatives of other countries invited by Her Majesty to be present in Westminster Abbey on the occasion of her coronation. It was held in the Great Hall of Westminster School. Sir David Eccles, the Minister of Works, paid us the unexpected compliment of asking us to undertake the luncheon, and then added ‘and to serve it also.’ Although we were simmering with excitement, R. H. and I let days go by before we allowed ourselves to make real plans, feeling that perhaps Sir David, remembering the youthfulness of our students, might modify his ideas about what we could accomplish. But nothing so discouraging happened.
Once the plan was firmly established we concerned ourselves exclusively with our problems, which in brief were these. The luncheon was for about three hundred and fifty people, the largest party to be seated in the Great Hall, the rest in a house some distance away. By two o’clock the guests would be very hungry and probably cold. There would be people of many nationalities, some of whom would eat no meat. Kitchen accommodation was too small to serve hot food beyond soup and coffee. The serving of the food would have to be simple because all the waitresses would be amateurs.
The menu chosen was as follows:

Potage de Tomates à l’Estragon (page 992)
Truite de Rivière en Gelée, Sauce Verte (page 507)
Poulet Reine Elizabeth (page 1012—Coronation Chicken)
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Cornets de Jambon Lucullus (page 999)
Cherry and Walnut Salad (page 1032)
Galette aux Fraises (page 1023)
Mousse au Citron (page 943)
Coffee, Petits Fours (page 876)

[page 1031]

[page 1032]
For Truite de Rivière the trout, boned, was coated in a delicately flavoured aspic, decorated with fresh dill, and accompanied by sauce verte, brown bread and butter, and sections of lemon.
Poulet Reine Elizabeth was chicken, boned, and coated in curry cream sauce, with, at one end of each dish, well-seasoned dressed salad of rice, green peas, and pimentos.
The Cornets de Jambon Lucullus were filled with a rich pâté.
The Cherry and Walnut Salad was chosen in place of green salad because it was thought too difficult to keep the latter perfectly fresh, since so much preparation had to be done ahead of time, and the dishes taken into the building overnight. The Cherry Salad recipe on page 352 was used, but with walnuts in place of almonds.

A U.S. PRECURSOR

It is worth noting that, in the USA, the excitement about the coronation of Queen Elizabeth II led to the creation, in April 1953, of a dish called coronation chicken.

This excitement was described in a United-Press article published in several U.S. newspapers in April 1953—for example in the Vermont Sunday News (Burlington, Vermont) of Sunday the 5^th:

New York—(UP)—Americans too poor to go to the British Coronation in June will get a touch of it anyway. The merchandise people are seeing to that.
Already you can buy everything from coronation candy to coronation carpets, from coronation dolls to coronation joke books. And retailers still have two months to go.
There are coronation salads, free trips to the event June 2, tied in with the promotion of some product such as coffee, and coronation tiaras for as little as 98 cents or as much as $50,000.
The nation’s poultry industry has gone all out. The poultry and egg national board, with headquarters here, has sent food writers a formal invitation—with the design of a crown on the cover—which reads:
“You are hereby commanded to attend the coronation of her majesty, Queen Chicken, on coronation chicken day, April 10.”

The first mention of a dish called coronation chicken that I have found is from the following, by the U.S. food columnist Edith Barber (1892-1963), published in several U.S. newspapers on Friday 24^th April 1953—for example in the Daily Press (Newport News, Virginia):

With everybody Coronation-conscious it is only fitting that the queen of the table should receive her due. Lady Chicken, who was “crowned” recently at an official dinner, has held first place in the food field for many a year.
Representatives of many nationally known food producers and distributors gathered to do her honor. Their offerings were admired (and tasted) by food editors at the “coronation” ceremonies. Among the variety of chicken dishes were “Coronation Chicken,” “Chicken Elizabeth,” “Chicken Royal” and “Chicken Regal.”

The dish itself was described in the following from The Capital Times (Madison, Wisconsin) of Thursday 30^th April 1953:

There comes a day in every homemaker’s life when she wants to serve a dish that is truly exotic in appearance, luxurious in flavor, and regal in almost glowing color.
That occasion may come when the boss and his wife come for their first meal, when the glamorous new neighbor is invited for luncheon, when the new bridge club meets and the girls come with their new finery, or a big birthday or family celebration rolls around.
The answer is the beautiful and delicious Coronation Chicken, which was crowned queen of chicken dishes two months before Queen Elizabeth dons the historic King Edward crown in Westminster Abbey. Its debut came when the Poultry and Egg National Board served it to many of the country’s leading food editors at “Coronation Chicken Day” ceremonies in New York.
Colorful as almost any dish can be, Coronation Chicken consists of a half pineapple, hollowed out and filled with a spicy Chicken a la King mixture, topped with Parmesan cheese, broiled a few minutes, and then surmounted with a “crown” made of an orange slice, cherries, and shredded coconut. It’s not only beautiful but almost a meal in itself.