Legally, a summons (also known in England and Wales as a claim form and in the Australian state of New South Wales as a Court Attendance Notice (CAN)) is a legal document issued by a court (a judicial summons) or by an administrative agency of government (an administrative summons) for various purposes.

A judicial summons is served on a person involved in a legal proceeding. Legal action may be in progress against the person, or the person's presence as witness may be required.[1] In the former case, the summons will typically announce to the person to whom it is directed that a legal proceeding has been started against that person, and that a case has been initiated in the issuing court.

http://en.wikipedia.org/wiki/Summons

--------------------------------------------------
Note added at 1 hr (2015-03-30 10:40:40 GMT)
--------------------------------------------------

Summons
The paper that tells a defendant that he or she is being sued and asserts the power of the court to hear and determine the case. A form of legal process that commands the defendant to appear before the court on a specific day and to answer the complaint made by the plaintiff.
http://legal-dictionary.thefreedictionary.com/summons