Wikipedia
A holy well or sacred spring is a spring or other small body of water revered either in a Pagan or Christian context, often both. Holy wells were frequently pagan sacred sites that later became Christianized. The term holy well is commonly employed to refer to any water source of limited size (i.e. not a lake or river, but including pools and natural springs and seeps), which has some significance in the folklore of the area where it is located, whether in the form of a particular name, an associated legend, the attribution of healing qualities to the water through the numinous presence of its guardian spirit or Christian saint, or a ceremony or ritual centred on the well site. In Christian legend, the water is often said to have been made to flow by the action of a saint, a familiar theme especially in the hagiography of Celtic saints.
Usage examples of "holy well".
It was found that daily resort was being made to a man named Hengst by a very large concourse of poor folk who had been bewitched, and with our own eyes we saw such crowds in the village of Eningen, that certainly the poor never flocked to any shrine of the Blessed Virgin, or to a Holy Well or a Hermitage, in such numbers as they went to that sorcerer.
I know not how, Morgaine, my love, but it comes to me that never again shall I drink of the Holy Well.
By the holy well of Mulhuddart I swear I'd pledge my chanza getting to heaven through Tirry and Killy's mount of impiety to hear it all, aviary word!