Billionth

Part of speech: noun

Pronunciation: /ˈbɪl.i.ənθ/

Definitions

1. A unit that signifies one among a billion total parts in a whole
2. The position in a sequence that corresponds to one part of a billion
3. A fraction equal to one divided by a billion ; the rank or order numbering one billion in a sequence

Etymology: The "-th" on "billionth" is the same ordinal ending found in "fifth" or "tenth," and it has been attached to "billion" to form the ordinal in English. The sense is simple: the position that matches one part of a billion, the way "millionth" marks one part of a million. The real story sits in the first half of the word. "Billion" was built in French rather than English, and its prefix "bi-" comes from a Latin word meaning "two," the same root that gives "bi-" in "bicycle" or "bisect." The construction followed the pattern of "million," which descends from Latin "mille," "thousand," with an augmentative ending added to make a larger unit. French writers took that pattern and created "billion" in the sense of a million million, a thousand million being a later and separate usage in English.