Coming last in a set of a billion terms: an ordinal numeral.
Being one of a billion equal parts.
n. One of a billion equal parts; the quotient of unity divided by a billion.
WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
adj. the ordinal number of one billion in counting order
n. one part in a billion equal parts
n. position 1,000,000,000 in a countable series of things
Word Usage
"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."