subpoena

     Subpoena is Latin for "under penalty", meaning a legal penalty. Someone who has been sent a subpoena has been ordered to appear in court to testify--to be a witness in a court case. The Constitution of the United States, Amendment VI, guarantees the right of the accused to have "compulsory process for obtaining witnesses in his favor", so the subpoena is not merely an instrument for the government. If the accused claims that he or she has an alibi, and that someone can bear witness to that alibi, the court will send a subpoena to force the person to appear in court--the same subpoena that would be sent to a witness for the prosecution.