The ordinal number matching the number 90 in a series.(noun)
One of 90 equal parts.(noun)
American Heritage(R) Dictionary of the English Language, Fifth Edition. Copyright (c) 2011 by Houghton Mifflin Harcourt Publishing Company. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.
Use "ninetieth" in a sentence
"In April 2010, shortly before his ninetieth birthday, Stevens announced his resignation after thirty-five years on the Supreme Court."
"Sitting in the airport before a television screen, I watch Nelson Mandela take the stage at his ninetieth birthday celebration."
"Approaching her ninetieth birthday, she felt that the job had aged her, that it had scooped her up and carried her summarily past her middle years—well, her late middle years."