A treehouse was a structure built in a tree. In case of Humans, small treehouses were usually built for a child's amusement.
Rain Robinson's next-door neighbors had a treehouse that could be seen through her brother's telescope. She related this to a time traveling Tom Paris in 1996. (VOY: "Future's End")
Aurelians residing on Starbase 25 lived in treehouses in the station's aviary. (LD: "An Embarrassment Of Dooplers")