Describing something in the position of number one billion in a sequence, or the fraction 1/1,000,000,000.
From 'billion' plus the ordinal suffix '-th,' following the same pattern as 'millionth' and 'thousandth.' Billion itself comes from French 'bi-' (two) plus '-llion,' originally meaning a million million.
The billionth is so incomprehensibly large that there's no billionth of a second measurement in standard physics—we use nanoseconds instead, showing how our language can create numbers our minds can't really grasp.
Complete word intelligence in one call. Free tier — 50 lookups/day.