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.
In most English-speaking countries, 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).
In most European countries, 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).