An Etymological Dictionary of Astronomy and Astrophysics

General: The act of uniting two or more things.
A number of persons, organizations, states, etc., joined or associated together for some common purpose, e.g. → International Astronomical Union (IAU).
Math.: A set containing all the elements of two or more sets being combined.
It is represented by ∪.

Etymology (EN): M.E. from O.Fr. union from L.L. unionem (nominative unio) “oneness, unity,” from unus, → one, cognate with Pers. yek, as below.

Etymology (PE): Yekâyeš, from yek, → one,

• -ây- epenthetic vowel, + -eš verbal noun suffix, → -tion.