Full moon
The moment the Sun and Moon are 180° apart in ecliptic longitude.
At a full moon Earth sits between the two and the Moon's lit face is turned fully toward us. It's the peak of the cycle, usually associated with things becoming visible — what was started a fortnight ago showing its result.
A full moon is a Sun–Moon opposition, so it carries the character of one: two ends pulling, a position to be found between them.
The sign is always opposite the Sun's: a full moon while the Sun is in Aries falls in Libra.