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n. 'atto-' is the standard SI prefix for multiplication by 10-18. A parsec (parallax-second) is 3.26 light-years; an attoparsec is thus 3.26*10-18) light years, or about 3.1 cm (thus, 1 attoparsec or microfortnight equals about 1 inch/sec). This unit is reported to be in use (though probably not very seriously) among hackers in the U.K.

See micro-.