The definition of 'cause'

The definition of ‘Cause’ - 

If the converse of a statement relating A and B is not necessarily true, then it (the pair A and B) is like a cause-effect pair. 

(This is since occurrence of a cause implies an effect, but the occurrence of the effect doesn't necessarily imply the occurrence of that cause).

If the converse of a statement relating A and B is necessarily true, then A and B are like equivalent events. 

(since, for example, equality of 3 sides of a triangle implies the equality of the 3 angles of the triangle and vice versa, make the 'equality of 3 sides' and the 'equality of 3 angles of a triangle' equivalent).

So 2 events being a cause-effect pair is the complement of them being equivalent.

So we can use the above statement to define a cause or an effect or a cause-effect pair as : 

If 2 events always accompany each other, and they aren't equivalent, then they are a cause-effect pair.