"Piste" is the French word for groomed trail or slope.
It has been adopted by North American skiers, and skiers in general, and most often combined with the word "off" - as in, "off-piste" - to describe backcountry skiing, or skiing on ungroomed, unmarked terrain.
Many ski words come from European languages, among them "schuss," "wedel" (pronounced vay-dul), "sitzmark," and, of course, "après ski."