Two celestial objects are said to be in opposition when they are positioned on opposite sides of the sky, as viewed from a location on Earth. When an object, such as a planet, is said to be “in opposition”, it usually means that the object is in opposition to the Sun.
Note that objects that orbit the Sun within the orbit of the Earth, such as the inferior planets (Mercury and Venus), cannot appear in opposition to the Sun.
Oppositions are a line-of-site effect, and the exact time of an opposition will depend on the observer’s location on Earth, due to the effect of parallax.
When an object is in opposition to the Sun, it will rise as the Sun sets and set as the Sun rises. Due to the fact that most planets have orbits with low eccentricities (i.e. they are roughly circular), this generally occurs when the planet is near to its closest approach to the Earth. Combined with the fact that the planet will appear fully illuminated by the Sun at opposition (analogous to a full Moon), as well as the “opposition effect” – a brightening of an planets surface due to it being illuminated from directly behind the observer – planets can appear particularly bright in the night sky when they are in opposition.
Note that both a conjunction and an opposition can also be referred to as a syzygy.