A lunar conjunction is the event when the earth, moon and sun, in that order, are approximately in a straight line. (See conjunction (astronomy) for a precise definition.) It is sometimes referred to as the new moon, though in Judaism, the new moon refers to observance by earth bound individuals of the first visible crescent of the moon. The period of time between two lunar conjunctions is the synodic month, which is a basic unit in most lunar (such as the Islamic) and luni-solar (such as the Hebrew) calendars.
The opposite event is called full moon or lunar opposition when sun, earth and moon, in that order, are approximately in a straight line.
References
Lunar conjunction Wikipedia(Text) CC BY-SA