In the common law, the peremptory pleas (pleas in bar) are pleas that set out special reasons for which a trial cannot go ahead. They are the plea of autrefois convict, the plea of autrefois acquit, and the plea of pardon.
A plea of "autrefois convict" (Law French for "previously convicted") is one in which the defendant claims to have been previously convicted of the same offence and that he or she therefore cannot be tried for it again. In the instance where a defendant has been summonsed to both criminal and civil proceedings, a plea of autrefois convict is essentially an application to 'merge' proceedings, giving rise to res judicata or a cause of action estoppel in civil proceedings. This means that the secondary proceedings should not go ahead and the complaint stopped. A plea of autrefois convict can be combined with a plea of not guilty.
A plea of "autrefois acquit" is one in which the defendant claims to have been previously acquitted for the same offence and thus should not be tried again. The plea of autrefois acquit is a form of estoppel by which the Crown cannot reassert the guilt of the accused after they have been acquitted. The plea prevents inconsistent decisions and the reopening of litigation.
Related doctrines include res judicata and, in the criminal context, a plea in bar of double jeopardy. (See Island Maritime Limited v Filipowski  HCA 30; 226 CLR 328). In the plea of pardon, the defendant claims that he or she has been pardoned for the offence and therefore cannot be tried for it.