A legal obstacle or barrier that prevents or destroys a legal action or claim, especially one that prevents the relitigation of an issue or the formation of a valid contract. See also double jeopardy, estoppel, merger, plea, and res judicata. 2 v.To prevent, prohibit, or act as a bar to. 3n.In bar.As a bar to an action. For example, if a defendant in a criminal action was acquitted earlier of the same charges that he is now accused, he may plead double jeopardy in bar. 4 adj. At bar. Now befo...