Malicious prosecution is considered an intentional tort under common law and means that a case that is taken against a defendant has no basis in law and should not have been taken in the first place.
If a lawsuit is only taken out of spite or to create the effect of taking a person to court for no reason, and the person is subsequently hauled through an unecessary court case, this would be considered a malicious prosecution.
In general, malicious prosecution is a wrongful action that has malicious intent. In order for a case to be classified as such, it must be determined to be in favor of the defendant and found to have no basis for being taken.
A person that brings a case that is determined to be a malicious prosecution will in turn be charged with a crime and sentenced accordingly.