action: a legal procedure brought against a party.

"Counsel for the shareholders commenced a legal action against the directors."

Preposition usage:
You bring an action against another party.
You bring an action for damages.
You bring an action before the court.
You can also file an action with the court.

Related words and concepts:
actionable: giving cause to a lawsuit. "You can't sue him just because he threw water at your car. If he had thrown paint, that would have been actionable."
cause of action: an actionable claim; the legal claims within a lawsuit. "The court dismissed his case for failure to state a cause of action."
class action: a type of collective lawsuit in which, for reasons of efficiency, a number of plaintiffs with a common legal position bring suit against the defendant or defendants. It is also known as a representative action. "A securities class action lawsuit has been filed on behalf of shareholders."
shareholders' derivative action: a lawsuit brought by shareholders to enforce the rights of the company. "When the company failed to collect on the outstanding debts, the shareholders brought a derivative action on the company's behalf."
civil action: a legal action, typically a lawsuit, to protect a private right or to compel a civil remedy as distinguished from criminal prosecution.