The garden aboard the Enterprise-D, a compartment of trees, flowers and a pond tended by the ship's botanists, used by the crew and their families for rest.
The arboretum is a planted garden inside the Enterprise-D, with paths, trees, flowering plants and a small pond. It is cared for by the ship's botanists, among them Keiko O'Brien before she leaves for Deep Space 9, and it gives a crew that includes families and children somewhere green to walk.
It appears in several episodes. In Imaginary Friend the young Clara Sutter plays there with Isabella, the invisible companion who turns out to be an alien presence studying the ship. In Dark Page it is beside the arboretum pond that Lwaxana Troi collapses while with the child Hedril, an event that leads Deanna to the buried memory of her drowned sister.
Gardens of this kind are found on other ships as well, and Voyager's airponics bay serves a more practical version of the same need.