full moon
Full moon occurs when the Moon is on the opposite side of the Earth from the Sun.
When it is directly opposite, it would be a Full Moon. But the Moon spends half of its orbit on the side opposite the Sun, which includes all of the Gibbous phases from First Quarter to Last Quarter.
The far side of the moon is always opposite of the visible side. When the moon is new, it is between the Sun and the earth. Since we see the dark side of the moon, the opposite side must be fully lit by the sun. Likewise when we see a gibbous moon, the far side would be a crescent. When we see a full moon, the far side is in complete darkness.
Venus and Mars
On the opposite side of the Earth from the Sun.
an eclipse
The Moon is on the opposite side of the Earth than the Sun is when there is a full Moon.
The same side of the moon always faces us, regardless of the phase. When the moon is full that side is fully illuminated by the sun as the moon is approximately on the opposite side of Earth relative to the sun.
a waxing gibbous
When the Moon is the opposite side of the Earth to the Sun.
To be full moon, the Moon has to be opposite the Sun - that way, we see the whole of the illuminated side. The only way for the moon to be on the opposite side of earth is for it to be on the night side. Thus the full moon cannot be out during the day.
This happens when the moon is on the opposite side of the earth from the sun. The moon reflects the light of the sun, and because it is directly opposite, we can see half of it from the earth. This is the half that appears as the full moon.