Definitions for "Billionth"
position 1,000,000,000 in a countable series of things
Keywords:  ordinal, counting, order, number, one
the ordinal number of one billion in counting order
One billionth is the reciprocal of a billion, which means it has one of two definitions depending on whether the long scale or short scale definition is being used.
Keywords:  part, one
one part in a billion