Alternative pleading is when a defendant, or other party in a lawsuit, makes two or more pleas instead of just one. The reason why this is done is because the defendant may be trying to discredit two or more different arguments for his or her guilt at the same time.
An example of an alternative pleading would be a defendant who pleads that he is not guilty for committing an assault because he was not at the scene of the crime, and also pleads not guilty by reason of insanity. In other words, the defendant is trying to make the point that not only was the defendant not only present at the scene of the crime, therefore ruling him out of responsibility for it, but the defendant is also insane. Insanity would also rule the defendant out of responsibility for the crime. This way, if the prosecution proves that the defendant was at the scene of the crime, the defendant still has a chance of being acquitted by reason of insanity.