1. the straight line configuration of 3 celestial bodies (as the sun and earth and moon) in a gravitational system
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.
2. (Gr. & L. Pros.) The coupling together of different feet;
as, in Greek verse, an iambic syzygy.
(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.
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
WordNet (r) 3.0 (2006):
n 1: the straight line configuration of 3 celestial bodies (as
the sun and earth and moon) in a gravitational system