In law, sua sponte (Latin: “of his, her, its or their own accord.”) describes an act of authority taken without formal prompting from another party. The term is usually applied to actions by a judge taken without a prior motionor request from the parties. The form nostra sponte (of our own accord) is sometimes used by the court itself, when the action is taken by a multi-member court, such as an appellate court, rather than by a single judge (third parties describing such actions would still refer to them as ‘sua sponte’). While usually applied to actions of a court, the term may reasonably be applied to actions by government agencies and individuals acting in official capacity.

One situation in which a party might encourage a judge to move sua sponte occurs when that party is preserving aspecial appearance (usually to challenge jurisdiction), and therefore cannot make motions on its own behalf without making a general appearance. Common reasons for an action taken sua sponte are when the judge determines that the court does not have subject-matter jurisdiction or that the case should be moved to another judge because of a conflict of interest, even if all parties disagree.

source of definition:

http://www.wikipedia.org