An opposition is the time when the Earth lies between a planet and the Sun, making the Sun and planet appear in opposite directions as viewed from the Earth. Oppositions are the best opportunities for observing superior planets (planets that orbit the Sun outside of Earth’s orbit) because they are well away from the glare of the Sun, and closer to the Earth than usual.
Strictly stated- its the arrangement of the Earth, Sun and planet when the apparent geocentric longitude of the planet and the Sun differ by 180°.
Note the photo above is from almost ten years ago when the Moon occulted Saturn.
Saturn currently sits in Virgo, but if you look up in the sky tonight, you can find it easily by looking under the tail of Leo the Lion.