An Etymological Dictionary of Astronomy and Astrophysics

Prefix denoting “two, both, twice.”

Etymology (EN): From L. bi-, cognate of Gk. di-, O.E. twi-; cf. Av. bi- “two,” biš “twice,” Mod.Pers. do “two,” PIE *dwo- “two.”

Etymology (PE): Do, Mid.Pers. do, Av. dva-, Skt. dvi-, Gk. duo, L. duo, E. two, Ger. zwei, Fr. deux.