The quincunx aspect in astrology is where two planets are one hundred and fifty degrees apart. This is an interesting aspect as it is an uneven division of the 360 degree circle. Unlike other aspects, it is not an aspect that produces an elegant number. This is very much the essence of this aspect. It is a difficult one.
The two planets involved forming a quincunx aspect, cannot easily see and communicate with each other. They are not straight in front of each other but slightly skewed off. Yet we know in some way they are related to each other. Since each sign is 30 degrees apart we can see from the one hundred and fifty degrees, that the planets are five signs away from each other. Unlike the trine aspects, the elements are not the same and the nature of the signs are also different (cardinal, fixed etc). This quincunx aspect is therefore a difficult one to deal with. It's rather like when you perform one action something related always happens at the same time yet you can't see a connection between the two events.
There is, however, a buried gift with the quincunx aspect, if you can finally get to see and understand the hidden connection between the planets involved. One could say the phrase 'opposites attract' is relevant. The planets are in such different signs and elements that in some way they hold the key to what the other planet does not have. They keep the other in check when it becomes too focused in one area. This is much like when you have an opinion, you immediately have a contrasting view that surfaces with it and in this way your understanding reaches a different level in the struggle to reconcile the two viewpoints.