An opposition is just that; when two things are opposite each other. It’s an astrological term for when two planets are opposite each other. This can refer to either when both planets are in transit, or when a transiting planet is making an opposition to one of the planets or points in your natal astrology chart - or even between two natal planets. Oppositions usually indicate that there’s a tug of war going on between the energy of those planets. It’s considered a challenging aspect, but not necessarily a bad one.
An opposition is exactly what it sounds like, but from a planetary standpoint it occurs at an angle of 180 degrees. This aspect is very similar to a conjunction, but where conjunctions unify energy, oppositions illustrate struggles. Oppositions also lead to mirroring qualities on occasion. If for example Mars and Uranus are in opposition on a single day, the planet that makes the largest movement in this aspect will mirror its qualities on the other.
Oppositions that occur in the charts are very similar. If you have a lot of oppositions, you may find that you frequently swing from some qualities to another, and frequently feel torn or distressed. On the other hand, if you have few oppositions in your chart, you will be the kind of person that goes with the flow, and experiences very few emotional disruptions in life.
Knowing and understanding where the oppositions are in your chart and karmic blueprint can help you see other perspectives, and enable you to make powerful decisions in your life where you control the destiny, rather than the planetary opposition.