tropical cyclone, nouna cyclone that originates over a tropical ocean area and can develop into the destructive storm known in the U.S. As a hurricane, in the western Pacific region as a typhoon, and elsewhere by other names.
Tropical Depression
A tropical cyclone in which the maximum sustained wind speed (using the U.S. 1 minute average standard) is up to 33 kt (38 mph, 17 m/s). Depressions have a closed circulation.
Tropical Storm
A tropical cyclone in which the maximum sustained surface wind speed (using the U.S. 1 minute average standard) ranges from 34 kt (39 mph,17.5 m/s) to 63 kt (73 mph, 32.5 m/s). The convection in tropical storms is usually more concentrated near the center with outer rainfall organizing into distinct bands.
Hurricane
When winds in a tropical cyclone equal or exceed 64 kt (74 mph, 33 m/s) it is called a hurricane (in the Atlantic and eastern and central Pacific Oceans). Hurricanes are further designated by categories on the Saffir-Simpson scale. Hurricanes in categories 3, 4, 5 are known as Major Hurricanes or Intense Hurricanes.
http://www.aoml.noaa.gov/hrd/tcfaq/A5.html
