An Etymological Dictionary of Astronomy and Astrophysics

A prefix meaning 10^-18.

Etymology (EN): From Danish or Norwegian atten “eighteen,” from O.N. attjan “eighteen,” from atta “eight” (compare with Gk. okto, L. octo, Skt. astau, Av. ašta-, Mod.Pers. hašt; PIE *okt(u))

• tjan “ten” (compare with Skt. dasa, Av. dasa, Mod.Pers. dah, Gk. deka, L. decem; PIE *dekm).