atto-

In units of measurement, a factor of 10^-18.

Danish or Norwegian atten, eighteen, with the ending changed to match that of the other SI (Système International) multiples (see the entry Words for multiples).

Such quantities are so tiny that they are only needed by scientists investigating extremely short time periods (attoseconds), or tiny forces (attonewtons). The term attoparsec has been used in fun to refer to a distance of slightly more than an inch (3.1 cm).