The description of the word you requested from the astronomical dictionary is given below.
oppositio = [Latin] opposite
Two celestial bodies are in opposition when they are in opposite directions in the sky (as seen from Earth). If only one celestial body is mentioned, then the second body is understood to be the Sun. Only planets or other celestial bodies that are further from the Sun than the Earth is can be in opposition (to the Sun). Around that time, they are above the horizon all night, and therefore the time of opposition is usually the best time to see the object. The opposite of an opposition is a conjunction.