Estoppel

That is; a party who has, either by his declaration or act, caused or permitted another to believe a thing to be true and to act upon such belief, neither he nor his representative in interest shall be allowed, in any proceeding between himself and such person or such person's representative in interest, to deny the truth of that thing. He must accept the new legal relationship as modified by his own words or action, whether or not it is supported by any point of law, or by any consideration but only by his word or conduct. See A.G. RIVER STATE V. A.G. AKWA-IBOM STATE (2011) 8 NWLR (pt. 1248) 31 SC.

This doctrine rests on the principle that there must be an end to litigation, and so, a Party must not be allowed to re-litigate a matter that has been settled in a final decision of a Court of competent jurisdiction. The estoppel created by a successful plea is a bar that prevents one from asserting a claim or right that contravenes what one has said or done before or what has been legally established as true; it is a bar that prevents re-litigation of issues – Black's Law Dictionary. 7th Ed.
For this principle to apply, certain pre-conditions must be present; such as that Parties must be the same in both the previous and present proceedings, and the claim or issues in dispute in both proceedings must be substantially the same – Alashe V. Ilu (1964) 1 All NLR 390.

However, I will add a few words to buttress the points made about estoppel per rem judicatum, which usually arises where an issue of fact has been judicially determined in a final manner between the Parties by a Court of competent jurisdiction, and the same issue comes directly into question in a subsequent proceeding between the same parties – Ntuks V. Nigerian Ports Authority (2007) 13 NWLR (Pt. 1051) 392.