Square
In plain words
Two planets a quarter of the way round from each other. They pull in directions that do not agree, so something has to give.
In the tradition's terms
90° apart: signs of the same mode but different element, which is why the friction has no shared ground to resolve on.
In depth
Separation 90°. Same quadruplicity, incompatible triplicity. The classical malefic angle, and the one most associated with events rather than moods.