Many legal systems throughout the world retain the use of Latin words or phrases that originated centuries ago in the legal system of ancient Rome. The term "suo moto" is one of those terms. Literally translated, it means "on its own motion," and the term generally refers to a situation wherein a judge acts without request by either party to the action before the court.
Under normal proceedings before a court, a judge's role is to direct the proceedings and act on motions filed. When a party to a court case wants the judge to rule on an issue or make a decision regarding something pertinent to the case, he files a motion with the court. The opposing party then has an opportunity to respond to the motion before the judge rules on it. In some cases, a judge acts suo moto, meaning without one of the parties' asking her to do so.
One common example of when a judge may act in this way is when she decides that she does not have personal or subject matter jurisdiction over the case before him or her. In other words, the judge decides that she does not have the legal authority to preside over the case. In that case, the judge may make a motion, without either party asking him or her to do so, moving the case to another jurisdiction or dismissing the case for lack of jurisdiction.
A judge may also act suo moto in a domestic case, such as a divorce or child custody case. Often, minor children are the subject of a domestic or family law case that comes before a judge. When the parents are battling over custody of the minor children and the judge feels that the best interest of the children is being overlooked, she may move to appoint a guardian ad litem. This person's role in the judicial system is to represent the best interests of the child throughout the proceedings.
In a criminal case, a judge may act on her own as well. She may decide that a jury instruction is necessary that was not requested by the parties, for example, or request that the jury members be polled to assure her that the verdict was unanimous.