the straight line configuration of 3 celestial bodies (as the sun and earth and moon) in a gravitational system
An alignment of three celestial bodies (for example, the Sun, Earth, and Moon) such that one body is directly between the other two, such as occurs at an eclipse.
The word syzygy comes from the Ancient Greek suzyugia, which means "union" or "yoke." It entered English in 1847 to describe an astronomical alignment.