A violent rotating windstorm that begins in the Southern Pacific and is characterised by the rapid inward circulation of air masses about a low-pressure center.
If a storm was to form in the Atlantic or eastern Pacific Ocean, we would call it a hurricane. So, basically every storm that endangers the United States is a hurricane.
A typhoon is generally any storm that begins in the western Pacific. So a storm that threatens Japan or the islands of Guam or the Philippines is called a typhoon. Winds from a typhoon are usually stronger than a hurricane, however they endanger far less land area due to their locations.