bridgeboard definition by Webster's New World
Webster's New World College Dictionary Copyright © 2010 by Wiley Publishing, Inc., Cleveland, Ohio. Used by arrangement with John Wiley & Sons, Inc.
any of the notched boards holding the treads and risers of a staircase; string
bridgeboard definition by American Heritage Dictionary
A notched board at either side of a staircase that supports the treads and risers.