It mainly depends on the strengthof its winds. The term to me seems to be ambiguous as sometimes tropical storms are referred to as Tropical Cyclones, Hurricanes,Tropical depression, etc. "The term "tropical" refers to both the geographic origin of these systems, which form almost exclusively in tropical regions of the globe, and their formation in Maritime Tropical air masses. The term "cyclone" refers to such storms' cyclonic nature, with counterclockwise rotation in the Northern Hemisphere and clockwise rotation in the Southern Hemisphere." http://en.wikipedia.org/wiki/Tropical_storm This site cites winds in such a storm as having a speed over 74 mph to qualify as a hurricane. "While all are large storms of tropical nature, their official storm name has to do with its wind speeds. When the storms are in their infant stages as just a general area of low pressure that has the potential to strengthen, they're usually given the name "Tropical Depression" If the storm's peak winds become greater than 39 mph, then it's now a "tropical storm" and it gets a real name from the National Hurricane Center, like "Tropical Storm Barry." ...If the storm continues to grow and reaches wind speeds over 74 mph, it's then a hurricane" http://www.komonews.com/weather/asksteve/4347506.html