Historically, the title comes from the Latin comes stabuli (attendant to the stables, literally count of the stable) and originated from the Roman Empire; originally, the constable was the officer responsible for keeping the horses of a lord or monarch.
The title was imported to the monarchies of medieval Europe, and in many countries developed into a high military rank and great officer of State (e.g. the Constable of France, in French Connétable de France, who was the commander-in-chief of all royal armed forces (second to the king) until prime minister Cardinal Richelieu abolished the charge in 1627).
Most constables in modern jurisdictions are law enforcement officers; in the United Kingdom, the Commonwealth of Nations and some Continental European countries, a constable is the lowest rank of police officer (it is also, when preceded by the term 'sworn', used to describe any police officer with arrest and other powers), while in the United States a constable is generally an elected peace officer with lesser jurisdiction than a sheriff. However, in the Channel Islands a constable is an elected office-holder at the parish level.
An equivalent position is that of Marshal, which derives from Old High German marah "horse" and schalh "servant", and originally meant "stable keeper", which has a similar etymology. (Considering the origin of the word "marshal", the title and rank of Air Marshal in the British Royal Air Force and others has a very interesting literal meaning.)
In Australia, as in the United Kingdom, constable is the lowest rank in most police services. It is often categorised into the following from lowest to highest: Probationary Constable, Constable, Constable First Class, Senior Constable, Leading Senior Constable. These variations depend on the individual State/Territory Police Force in question.
Senior Constable generally refers to a police officer of the rank above Constable and is denoted by way of two chevrons/stripes.