The blossom of the elderberry, often specifically the European species Sambucus nigra(noun)
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Use "elderflower" in a sentence
"A few days later, when I had joined her at her father's house, Jackie and I made a batch of the kind of elderflower cordial/syrup I recently used in a gooseberry relish (see here): equal volumes of water and sugar brought to the boil to form a simple syrup"
"Visitors can also try croquet - a popular garden activity in bygone times - and they can even sample period refreshments such as elderflower cordial or fruit wine."
"Inside, would be a floating island, made from meringue, cast in a pool of cold elderflower soup, topped with goat cheese ice cream and perhaps some caramelized walnut powder."