In units of measurement, a factor of 10-12.
[Spanish pico, a little bit.]
This is one of the standard SI (Système International) multiples (see the entry Words for multiples). An example is picosecond, a millionth of a millionth of a second; others are picogram, picofarad, and picocurie. A rare example in the looser sense of ‘extremely small’ occurs in picocell, a very small area of coverage in a wireless communication system, perhaps only a single room within a building.