1. Dwelling built among the branches of trees, affording shelter from floods, wild animals, etc.
2. Folly, or ornamental summer-house built in European gardens from C16 (e.g. the C18 Gothick example at Pitchford Hall, Salop.). If (1) had ever been a precedent for (2), the evidence is elusive.
"tree-house." A Dictionary of Architecture and Landscape Architecture. . Encyclopedia.com. (August 17, 2018). http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/tree-house
"tree-house." A Dictionary of Architecture and Landscape Architecture. . Retrieved August 17, 2018 from Encyclopedia.com: http://www.encyclopedia.com/education/dictionaries-thesauruses-pictures-and-press-releases/tree-house