Source : Webster's Revised Unabridged Dictionary (1913)

Syzygy \Syz"y*gy\ (s[i^]z"[i^]*j[y^]), n.
   The intimately united and apparently fused condition of
   certain low organisms during conjugation.

Syzygy \Syz"y*gy\ (s[i^]z"[i^]*j[y^]), n.; pl. {Syzygies}
   (-j[i^]z). [L. syzygia a joining together, conjunction, Gr.
   syzygi`a; sy`n with + zeygny`nai to join, zygo`n yoke: cf. F.
   syzygie. See {Yoke}, n.]
   1. (Astron.) The point of an orbit, as of the moon or a
      planet, at which it is in conjunction or opposition; --
      commonly used in the plural.

   2. (Gr. & L. Pros.) The coupling together of different feet;
      as, in Greek verse, an iambic syzygy.

   3. (Zo["o]l.)
      (a) Any one of the segments of an arm of a crinoid
          composed of two joints so closely united that the line
          of union is obliterated on the outer, though visible
          on the inner, side.
      (b) The immovable union of two joints of a crinoidal arm.

   {Line of syzygies} (Astron.), the straight line connecting
      the earth, the sun, and the moon or a planet, when the
      latter is in conjunction or opposition; -- used chiefly of
      the moon.

Source : WordNet®

syzygy
     n : the straight line configuration of 3 celestial bodies (as
         the sun and earth and moon) in a gravitational system