Source : Webster's Revised Unabridged Dictionary (1913)

Roller coaster \Roller coaster\
   An amusement railroad in which cars coast by gravity over a
   long winding track, with steep pitches and ascents.

Source : WordNet®

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: {big dipper}, {chute-the-chute}]