Syzygy
When three or more celestial bodies are in alignment, the phenomenon is known as syzygy. Common syzygy occurrences include full moons (the sun and moon are in opposition relative to the earth), new moons (the sun and moon are in conjunction - i.e. aligned with each other relative to the earth and on the same side) and eclipses. When two planets (or one planet and the moon) are in conjunction, that too is syzygy (as our ability to observe the alignment of the planets imply the earth too is on the same line).