The ordinal number matching the number one billion in a series.(noun)
One of a billion 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 "billionth" in a sentence
"In the meantime, a few weeks ago, I watched The Sound of Music (for the billionth time)."
"When it was announced a couple of weeks ago that Flickr, the photo-hosting site, had hosted its five billionth picture, someone pointed out smugly that Facebook already has over three times that number."
"There is much significance made of the "first living cell", but the simplest replicating units discovered so far are actually ribozymes (RNA enzymes) which have less than a billionth as many atoms as a cell (a cell is a few quadrillion atoms, while a ribozyme is a few tens of thousands, if I understand correctly)."