The opposition is the simplest aspect to find and the hardest to soften. A hundred and eighty degrees halves the circle, and the signs it joins are always of the same modality and of complementary elements: fire opposes air, earth opposes water. Every sign has exactly one opposite, and the pairs are fixed, so the axis a planet stands on can be named before anything else about the chart is known.
Counting it by sign and counting it by degree almost always agree, which is unusual. Because the opposition spans a full six signs, a planet early in one sign and a planet late in its opposite are still within a generous orb of the exact angle in most systems, so the sign-based and degree-based readings rarely part company the way they do for the square or the sextile. The practical disputes about the opposition are therefore not about whether it is present but about what it does.
Traditional sources place it with the square among the configurations of enmity and pair it with obstruction, open conflict and things that come to a person from outside. Modern psychological practice reads the same angle as a demand for integration between two functions that are each incomplete, and treats projection, the attribution of one end of the axis to another person, as its characteristic mechanism. That reading belongs to the twentieth century and to the psychological turn in astrology generally; it is not what an ancient source means by the term. Both are in circulation, and they lead to different advice.