emergency braken. See parking brake.American Heritage® Dictionary of the English Language, Fifth Edition. Copyright © 2016 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.emer?gency brake` n. a hand- or pedal-operated brake used to prevent a motor vehicle from rolling, esp. after it has been parked. [1895?1900] Random House Kernerman Webster’s College Dictionary, © 2010 K Dictionaries Ltd. Copyright 2005, 1997, 1991 by Random House, Inc. All rights reserved.