A pied-à-terre is a place of lodging for occasional or secondary use. In French, the term literally means "a foot on the ground." It was named such because basic crash pads used by traveling execs during the week were traditionally on the ground level of apartment buildings. Nowadays, pieds-à-terre are often luxury city homes for couples whose kids are grown and want to escape the 'burbs, and interior designed second homes in international cities for the jet-setting type.
Source