The American Heritage® Dictionary of the English Language, 4th edition Copyright © 2010 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
- Not marked with or showing a date: an undated letter; an undated portrait.
- Being such as to have long-lasting appeal without ever going out of style: a classic, undated dress.