Answer: According to the New York Botanical Garden's Illustrated Encyclopedia of Horticulture's entries under "jonquil" and "daffodil":
"Most appropriately jonquil is restricted as a vernacular name to slender, rush-leaved Narcissus jonquilla and its hybrids, but in parts of the U.S., including New York City and its environs, other narcissuses, especially large-trumpet daffodils, are known as jonquils."
Daffodil "is a popular and old vernacular name for large-trumpet varieties of Narcissi. It is sometimes used loosely and less accurately for short-trumpet kinds. In some parts of North America, including New York, daffodils are often called jonquils. This is regrettable because the name properly should be reserved for Narcissus jonquilla and its varieties and hybrids."
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