Astronomical opposition

Opposition is a term used in positional astronomy and astrology. It means that, as seen from some place (usually the Earth), one celestial body is opposite to another in the sky.

Specifically, two planets are in opposition to each other when their ecliptic longitudes differ by 180°.

A planet (or asteroid or comet) is simply said to be in opposition, when it is in opposition to the Sun, as seen from the Earth. This is the best time to observe a planet:

it is visible almost all night (it rises around sunset, culminates around midnight, and sets around sunrise);
it makes its closest approach to the Earth during its orbit around this time, so it appears biggest and brightest.

The Moon (which orbits the Earth rather than the Sun) is in opposition to the Sun at full moon.