summerhouse

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A small, roofed structure in a park or garden affording shade and rest; a gazebo.

from The Century Dictionary.

• noun A structure in a park or garden, sometimes elaborate, but move often of the simplest character, generally little more than a roof supported on posts, and with the sides open or closed merely with a lattice for the support of vines, intended to provide a shady and cool place to sit in the open air, or for the enjoyment of a view, or the like. Compare kiosk and pavilion.
• noun A house for summer residence.

from the GNU version of the Collaborative International Dictionary of English.

• noun A rustic house or apartment in a garden or park, to be used as a pleasure resort in summer.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A house owned not as a primary residence and used during warm weather months of the year.
• noun A house designed to be cool and used as a residence only during warm weather.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun a small roofed building affording shade and rest
• noun a country house (usually located in the country) that provides a cool place to live in the summer