Pico-

Pico- (symbol p) is a unit prefix in the metric system denoting one billionth, a factor of 10−12 (0.000 000  000  001).

Derived from the Spanish pico, meaning peak, beak, bit, this was one of the original 12 prefixes defined in 1960 when the International System of Units was established.

The radius of atoms range from ~25 picometers (hydrogen) - 260 picometers (caesium). One picolight-year is about nine kilometers (six miles).