Elderflower pressé is a sweetened, carbonated soft drink.
It is made from elderflowers, the flowers of the European elderberry, Sambucus nigra, which are also used to make elderflower cordial, or a synthetic equivalent.
[1][2] Other ingredients will include lemons and sweetener.
Elderflower pressé is therefore similar to citron pressé and lemonade, and effectively a premixed form of elderflower cordial.
Like them, it is used as a mixer in cocktails.