two celestial bodies are said to be in opposition when they are on opposite sides of the sky, viewed from a given place (usually Earth)
Opposition is a word used in observational astronomy (and also astrology). It is used when two objects in the sky are on the opposite side of the sky when viewed from the Earth.
In particular, two planets are in opposition to each other when their ecliptic longitudes differ by 180°. When talking about a single object being in opposition, it is assumed to be opposite from the sun.