In astronomy, a syzygy is a situation where three bodies are situated along a straight line. It is usually used in context with the sun, earth, and the moon or a planet, where the latter is in conjunction or opposition. For example, solar and lunar eclipses are examples of syzygies.

The term is also often loosely used to describe interesting configurations of planets in general. For example, situations when all the planets are on the same side of the sun are sometimes called syzygies, although they are not necessarily found along a straight line.

In psychology, Carl Gustav Jung used the term syzygy to denote an archtypal pairing of contrasexual opposites, which symbolized the communication of the conscious and unconscious minds.