Rooftop pavilions intended as lookouts or for the enjoyment of a view. For unroofed platforms, use "widows' walks"; for rooftop structures that are primarily ornamental, use "cupolas," for small pavilions, in a garden setting, intended for enjoyment of a view, use "gazebos."

Source: Art and Achitecture Thesaurus

http://vocab.getty.edu/page/aat/300070138