roller coaster

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A steep, sharply curving elevated railway with small open passenger cars that is operated at high speeds as a ride, especially in an amusement park.
• noun An action, event, or experience marked by abrupt, extreme changes in circumstance, quality, or behavior.

from the GNU version of the Collaborative International Dictionary of English.

• 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.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun Alternative spelling of rollercoaster.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun elevated railway in an amusement park (usually with sharp curves and steep inclines)
• noun anything characterized by abrupt and extreme changes (especially up and down)