Series of steps, to be ascended or descended, that wind around a central point. Often, this central point constitutes of a column made of metal or wood.
Building a spiral staircase is no mean feat. From http://jself.com/stair/Stair.htm:
When you enter a spiral staircase you'll revolve about a full circle and be directly above or below your starting point. If you're building your staircase from the edge of a loft, or through a floor opening, exit and entry shouldn't be a problem. However, if you're planning a corner installation, or against a wall, pay particular attention to direction of rotation and exiting problems that may develop. If the bottom tread is in a corner you may not be able to walk out from under the staircase without ducking your head.
All spiral staircases rotate as they climb. The amount of rotation is determined by tread number and width. If the staircase rotates too slowly (narrow treads) it will not have sufficient step area for safe walking. You would be making something resembling a twisted ladder, which may be impossible to descend safely. When the staircase treads are too wide, it will rotate so quickly that it's returned under itself limiting your headroom. When the top landing is too wide it will also limit headroom under the landing.
A surefire way to tangle people up in knots is to ask them to describe a spiral staircase without making use of their hands for illustration.