1A room in a house, or a building (especially an ornamental one) in a garden, with a door or window opening out on to or providing a view of the garden.
2Space in a garden, especially that available for planting.
3A section of a garden viewed as a discrete space, typically defined by plants, hedges, etc., especially one designed to capture a particular theme or feel, or to serve a particular purpose.
Early 17th century; earliest use found in Two Wise Men & All Rest Fooles. From garden + room.