The Collaborative International Dictionary of English v.0.48:

Roller coaster \Roller coaster\
   An amusement railroad of varying design in which open cars
   coast by gravity over a long winding track in a closed
   circuit, with steep pitches and ascents, and in some cases
   loops in which the cars are briefly upside-down; typically,
   the cars are pulled by a chain device to the top of the first
   peak, after which gravity and momentum provide the only
   propulsive forces. In some cases, the cars are suspended from
   a monorail rather than resting on a track, and such cars may
   be made to swing outward at an angle near to the horizontal.
   It is a popular amusement at many amusement parks, but is
   sufficiently frightening to some people that they refuse to
   ride in one.
   [Webster 1913 Suppl. +PJC]

WordNet (r) 3.0 (2006):

roller coaster
    n 1: anything characterized by abrupt and extreme changes
         (especially up and down); "the economy has been on a roller
         coaster all year"
    2: elevated railway in an amusement park (usually with sharp
       curves and steep inclines) [syn: roller coaster, big
       dipper, chute-the-chute]