Русско-англо-русский юридический словарь

A plaintiff (Π in legal shorthand), also known as a claimant or complainant, is the term used in some jurisdictions for the party who initiates a lawsuit (also known as an action) before a court. By doing so, the plaintiff seeks a legal remedy, and if successful, the court will issue judgment in favor of the plaintiff and make the appropriate court order (e.g., an order for damages). [...]

In Hong Kong, Canada and the United States, as well as in both Northern Ireland and the Republic of Ireland, the legal term "plaintiff" is still in use. American usage traditionally limits the application of terms such as "claimant" and "claim form" to extrajudicial process in insurance and administrative law. After exhausting remedies available through an insurer or government agency, an American claimant in need of further relief would turn to the courts, file a complaint (thus establishing a real court case under judicial supervision), and become a plaintiff. (Wikipedia)

In other words, if a plaintiff initiates a lawsuit and a defendant responds to the lawsuit with claims of their own against the plaintiff, the defendant’s claims are “counterclaims.” (Wikipedia)

A motion is a written or oral application to a court in a pending case seeking some sort of ruling or order. A petition, on the other hand, is always in writing, and is considered a pleading, used to commence a proceeding, or initiate a collateral one. (Avvo)

It is not clear who initiates these bans, let alone why, but President Berdimuhamedow is a keen driver and pays particular attention to traffic regulations. (BBC)