Lunation
A lunation is a New or Full Moon, the two points where the Moon meets or opposes the Sun, and the lunation cycle is the twenty-nine and a half days between one New Moon and the next.
The New Moon is a conjunction of Sun and Moon and the Full Moon their opposition, so the whole cycle is one aspect widening and closing again. The quarters are the squares between them.
Traditional practice reads the waxing half as the time for building and the waning half for finishing and clearing, and electional astrology has used that division for centuries. It is a rhythm rather than a rule.
Frequently asked
How long is a lunation cycle?
About 29.5 days from one New Moon to the next, which is where the length of a calendar month comes from.
Is a Full Moon the same as an eclipse?
Only when it falls close to a lunar node. Most Full Moons miss the alignment and pass without an eclipse.