atto-

1. a combining form that enters into compounds denoting a unit one-quintillionth (10−18) the size of the unit named by the stem.

prefix

1. denoting 10 –18attotesla Symbol: a

word-forming element meaning “one quintillionth,” 1962, from Danish atten “eighteen” (a quintillion is 10 to the 18th power), related to Old English eahtatene (see eighteen).

pref.

1. One quintillionth (10-18):attometer.