A magistrate is a judicial officer exercising some of the functions of a lower-level judge. In federal court, a magistrate judge is a judicial officer of a district court who conducts initial proceedings in criminal cases, decides criminal misdemeanor cases, conducts many pretrial civil and criminal matters on behalf of district judges, and decides civil cases with the consent of the parties.
More generally, a magistrate in English common law is a low ranking judge who sits in a magistrates court. Magistrates are normally not full-time and serve voluntarily as a service to the community although they may be reimbursed for any out of pocket expenses. Magistrates courts are limited in their sentencing powers to a maximum of six months jail and can only try lesser offenses, including most motoring offenses. They do not have juries, but instead are presided over by a panel of three magistrates who determine the guilt or innocence of the accused. Defendants may have the option of selecting a jury trial before a judge in more serious cases, but they may also face a more severe sentence if found guilty if they choose a jury trial. Magistrates courts operate in England, Hong Kong and British Overseas Territories among other places.
"Magistrate" is also the title given to the head of a county in China and Taiwan. During imperial times, these people exercised a judicial, as well as an executive function. However, in modern times, they no longer have judicial functions. Rather they function as head of the county government. The Chinese term for this person is "xianzhang" (县长), whereas in Hong Kong, the magistrates referred to in the first paragraph are called "caipan" (裁判).
In Idaho, the trial level judges are referred to as Magistrates. Unlike their namesake, they have full judicial power at the trial level. The US District Courts also have judicial officers who serve for eight year terms known as magistrates, however they are limited in their powers.