Word for the day: syzygy

ˈsɪzɪdʒɪ

  1. Astron. a.† Orig. = CONJUNCTION 3. Obs.
    b. Now extended to include both conjunction and opposition (Opposition 3) of two heavenly bodies, or either of the points at which these take place, esp. in the case of the moon with the sun (new and full moon). Often opposed to quadrature 4b, c.
  2. Anat. pl. The pairs of cranial nerves. Obs.
  3. Math. A group of rational integral functions so related that, on their being severally multiplied by other rational integral functions, the sum of the products vanishes identically; also, the relation between such functions.
  4. A pair of connected or correlative things; in Gnostic theology, a couple or pair of opposites, or of æons.

OED

I encountered this beauty of a word unfortunately in its fifth meaning, but it should have plenty of uses.