The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. (Gr. & L. Pros.) The coupling together of different feet;
      as, in Greek verse, an iambic syzygy.
      [1913 Webster]

   3. (Zool.)
      (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.
          [1913 Webster]

   4. The intimately united and apparently fused condition of
      certain low organisms during conjugation.

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

syzygy
    n 1: the straight line configuration of 3 celestial bodies (as
         the sun and earth and moon) in a gravitational system