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.
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Short scale
In the short (or American) scale, a billionth is equal to 0.000 000 001, or 1 x 10−9 in scientific notation. The prefix for this number is nano, and is abbreviated as "n" (for example, in electronics, one nanofarad would be written as 1 nF).
Long scale
In the long (or English) scale, a billionth is equal to 0.000 000 000 001, or 1 x 10−12 in scientific notation. The prefix for this number is pico-, and is abbreviated as "p" (for example, in electronics, one picofarad would be written as 1 pF).
References
Billionth Wikipedia(Text) CC BY-SA