1. The ordinal number of one billion in counting order
   "The billionth digit of pi was calculated by a supercomputer"

1. Position 1,000,000,000 in a countable series of things
   "The billionth grain of sand on the beach"
    
2. One part in a billion equal parts
   "A nanosecond is one-billionth of a second";
   - one-billionth

Derived forms: billionths

See also: ordinal

Type of: common fraction, rank, simple fraction, vulgar fraction [Brit]

Encyclopedia: Billionth