Which term is used to describe actions taken without any formal complaint or notification?

The term that describes actions taken without any formal complaint or notification is "sua sponte." This Latin phrase translates to "of one’s own accord" and is often used in legal contexts to refer to actions that a judge or court takes on its own initiative, without a request from either party involved in a case. For instance, a judge may decide to address an issue or make a ruling based on what they perceive as an important matter for the court, even if the parties have not formally brought it up.

This contrasts with the other terms listed. "Supra" is used in legal writing to refer to something that has been mentioned earlier in the text, primarily for citation purposes. "Viva voce" refers to oral evidence or testimony given in person, as opposed to written statements, and is not connected to the initiation of actions by the court. "Stare decisis," on the other hand, relates to the legal principle of adhering to precedent and making decisions based on prior rulings. Each of these terms has a different application in the legal field, making "sua sponte" the correct choice for the concept of initiating actions without formal notification.