It is the condensate water that remains after the extraction of an essential oil by water or steam distillation. When essential oils are produced this way, not all of the aromatic and healing principles held within the plant are actually captured in the essential oil.
Certain components are hydrophilic, which means they dissolve into water, and this produces what is variously known as a flower water, hydrosol or hydrolate.
In other words: floral waters are distilled water infused with the essence of flowers. For that reason they are often known as essential waters.