When the Moon is directly in line with the Sun, it's called a syzygy. This term is used for any time when three celestial bodies align, but it's most commonly used for the Moon and Sun.

There are two specific syzygies involving the Moon and Sun:

* New Moon: This occurs when the Moon is between the Earth and the Sun. The Moon's illuminated side is facing away from Earth, making it invisible to us.

* Full Moon: This occurs when the Earth is between the Moon and the Sun. The Moon's illuminated side is fully facing Earth, making it appear completely bright.

So, while "syzygy" is the most accurate term, it's important to consider which specific alignment you're referring to (New Moon or Full Moon).