one-time pad (w?n?t?m?)n. A key used in private-key cryptography that is the same length as the contents of the message being encrypted, providing unbreakable encryption if the key is perfectly random and used only once.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.