The sacred groves* of the Druids, priests of the Celtic religion.** Druids, according to the Roman writers Pliny, Lucan, etc., did not meet in stone temples, or any permanent structure, but met in the woods, in sacred groves of trees. The tree considered most sacred was the oak; to find mistletoe on an oak was cause for much celebration.
Nemetona was a Gaulic goddess, the personification of the sacred grove.
*The term "grove" now refers to a group of druids, equivalent to a church, lodge, or coven. It no longer only means "a group of trees."
**Here called Druidism for convenience's sake; I have problems with that, but see the w/u for my explination).