Wiktionary
n. An area in a park type setting, often within a ski resort, designed for snowboarding or sometimes skiing and typically containing features such as ramps and rails for sliding on.
Wikipedia
A terrain park is an outdoor recreation area containing terrain that allows skiers, snowboarders and snowbikers to perform tricks. Terrain parks have their roots in skateparks and many of the features are common to both.
From their inception to as recently as the 1980s, ski areas generally banned jumping and any kind of aerial maneuvers, usually under penalty of revoking the offender's lift ticket. By the 1990s, most areas provided snow features specifically catering to aerial snowsports. One of the first in-bounds terrain parks was the snowboard park built in 1990 at Vail's (Colorado) resort. The park was copied soon in other resorts. Today most resorts have terrain parks, with many having multiple parks of various difficulty. Some resorts are almost exclusively terrain parks such as Echo Mountain Park in Idaho Springs, Colorado and Snow Park in Wanaka, New Zealand. In Colorado there has been a recent trend for defunct resorts such as Squaw Pass (now Echo Mountain Park) to be reopened, catering to terrain park users.