Glossary
Petitioner
The party who starts certain kinds of cases: typically family law cases (divorce, custody), probate cases (wills, estates), or appeals. Functionally similar to a plaintiff, but the term used depends on the type of case.
Different courts use different words for the same role. In a typical civil lawsuit you have a plaintiff and defendant. In a divorce case, family court, or probate matter, the same role is called a petitioner and the other side is a respondent. In an appeal, the side asking the higher court to review the case is the appellant (or sometimes also called the petitioner).
The word changes; the role doesn't. The petitioner is the one who started the case and is asking the court to do something.