"conjunction or opposition of a heavenly body with the sun," 1650s, from Late Latin syzygia, from Greek syzygia "yoke of animals, pair, union of two, conjunction," from syzygein "to yoke together," from assimilated form of syn- "together" (see syn-) + zygon "yoke" (from PIE root *yeug- "to join"). Related: Syzygial; Syzygiacal; Syzygetic.

Proto-Indo-European root meaning "to join."

It forms all or part of: adjoin; adjust; conjoin; conjugal; conjugate; conjugation; conjunct; disjointed; enjoin; injunction; jugular; jostle; joust; join; joinder; joint; jointure; junction; juncture; junta; juxtapose; juxtaposition; rejoin (v.2) "to answer;" rejoinder; subjoin; subjugate; subjugation; subjunctive; syzygy; yoga; yoke; zeugma; zygoma; zygomatic; zygote.

It is the hypothetical source of/evidence for its existence is provided by: Sanskrit yugam "yoke," yunjati "binds, harnesses," yogah "union;" Hittite yugan "yoke;" Greek zygon "yoke," zeugnyanai "to join, unite;" Latin iungere "to join," iugum "yoke;" Old Church Slavonic igo, Old Welsh iou "yoke;" Lithuanian jungas "yoke," jungti "to fasten to a yoke;" Old English geoc "yoke."