What is the difference between terminology used in legal actions that refer to the parties in an action?

Various terminology is used in legal actions to refer to parties, such as, “plaintiff/petitioner/complainant/claimant/defendant/appellant/respondent..."  So what do they mean? 

There are subtle differences between the use of these terms. 

Plaintiff : A Plaintiff is usually the person who files a legal action against a Defendant. 

Defendant: A Defendant is usually the person who is alleged to have engaged in unlawful conduct by the Plaintiff. 

Claimant: An arbitration is an alternative to a trial in a courtroom.  The Plaintiff is referred to in arbitration as the Claimant.  Petitioner: A Petitioner is typically the person who files a petition against a Respondent seeking some sort of immediate relief from a tribunal. 

Respondent: A Defendant in an arbitration is referred to as a Respondent.  However, a Respondent can be either a Plaintiff or Defendant in an appeal from the lower court, depending on who won the case.  A person becomes a Respondent when the losing party (Appellant) from the initial case appeals from the decision of the lower court.

Appellant: the party initiating an appeal. 

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