What is the defence of res judicata?

In Ord v. Ord, [1923] 2 K.B. 432 (D.C.), Lush J. discussed what he referred to as two aspects of the defence of res judicata, a narrow sense and, at p. 443, “the wider principle” which permits the application of the defence where new facts are brought forth which could and should have been dealt with in the same action. He stated: “It remains for me to deal with the other, the wider principle to which I have referred and which is often treated as falling within the plea of res judicata. The maxim “nemo debet bis vexari” prevents a litigant who has had an opportunity of proving a fact in support of his claim or defence and chosen not to rely on it from afterwards putting it before another tribunal. To do that would be unduly to harass his opponent, and if he endeavors to do so he would be met by the objection that the judgment in the former action precluded him from raising that contention. It is not that it has already been decided, or that the record deals with it. The new fact has not been decided; it has never been in fact submitted to the tribunal and it is not really dealt with by the record. But it is by reason of the principle stated, treated as if it had been.”