A 'catio.' But i would want some grass in mine.
CAT ENCLOSURE. DIY project to keep your cats safe. Definitely an interesting idea.
Cat Walk http://www.ehow.com/ehow-extras/blog/14-photos-of-fancy-cat-furniture-and-tips-on-how-to-recreate-the-designs/
CatsOnDeck.com offers custom-built catios (or buy the pieces and build yourself).
cat run catio
Catios: The fanciest outdoor cat enclosures look like an extension of the house, or as cats call them, indoor human enclosures. I'm convinced I need one for Jack.
Cat Enclosures And Catios, Outdoor Cat Enclosures - Catio Spaces - Seattle, Wa