"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" (see jugular). Related: Syzygial; Syzygiacal; Syzygetic.