/a-(ˌ)tōpär-ˌsek/
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 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-.