What is Peremptory Plea?

Legal Definition
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 [2006] 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.
-- Wikipedia
Legal Definition
"Pleas in, bar" are so termed in contradistinction to that class of pleas called "dilatory pleas." The former, viz., peremptory pleas, are usually pleaded to the merits of the action, with the view of raising a material issue between the parties; while the latter class, viz., dilatory pleas, are generally pleaded with a view of retaining the plaintiff's proceedings, and not for the purpose of raising an issue upon which the parties may go to trini and settle the point in dispute. Peremptory pleas are also called "pleas in bar," while dilatory pleas are said to be in abatement only. Brown.
-- Black's Law Dictionary
Legal Definition
Pleading. A plea which denies the plaintiff's cause of action. 3 Bouv. Inst. n. 2891. Vide Plea.
-- Bouviers Law Dictionary
Legal Definition
A plea setting up a peremptory defense. See Peremptory defense.
-- Ballentine's Law Dictionary