Elderflower is a flower of the Elder shrub or trees which bears fruits and berries and found mostly in Europe and more generally in the Northern hemisphere. Elderflower is used to produce Elderflower Cordial and champagne. Elder trees were supposed to ward of evil spirits and protects against them. Elderflower cordial is produced by steeping Elderflower heads into a concentrated sugar solution so that the flavor is infused into the sugar syrup. The sugar concentration allows the cordial a long life.
The Nectar itself, tastes mostly of Apple juice with a bit of rose water (or what I now know to be Elderflower Cordial), adding character to the tried and tested Apple nectar. The former combination would be my guess in a blind test or perhaps if I were to re-invent the flavor, without having to go through the trouble of sourcing Elderflower Cordial. Maybe its just my taste buds!!