Informal usage varies beyond its technical definition to refer to any "just cause" a nation may claim for entering into a conflict. As such, it has been used both retroactively to describe situations in history before the term came into wide usage and in the present day when describing situations when war has not been formally declared.
Formally, a government would lay out its reasons for going to war, as well as its intentions in prosecuting it and the steps that might be taken to avert it. In so doing, the government would attempt to demonstrate that it was going to war only as a last resort (ultima ratio) and that it in fact possessed "just cause" for doing so.
No quarter, war to the death! a pretty woman is a casus belli; a pretty woman is flagrant misdemeanor.
Victor Hugo -- Les Miserables
Perhaps we can prevent the establishment of a German naval base to the south not by declaring war on Turkey, which seems rather a roundabout way of doing it, but by informing the Germans that it would be casus belli.