From AMS Glossary
A cyclone in tropical or subtropical latitudes (from the equator to about 50°N) that has characteristics of both tropical cyclones and midlatitude (or extratropical) cyclones.
They occur in regions of weak to moderate horizontal temperature gradient and extract the associated available potential energy, as do baroclinic cyclones, but they also receive some or most of their energy from convective redistribution of heat acquired from the sea, as do tropical cyclones. These storms usually have a radius of maximum winds that is larger than what is observed in purely tropical systems, and their maximum sustained winds have not been observed to exceed about 32 m s-1 (64 knots). Subtropical cyclones sometimes become true tropical cyclones, and likewise, tropical cyclones occasionally become subtropical storms. Subtropical cyclones in the Atlantic basin are classified by their maximum sustained surface winds: Subtropical depressions have surface winds less than 18 m s-1 (35 knots), while subtropical storms have surface winds greater than or equal to 18 m s-1. While these storms are not given names, forecasters do issue warnings for them.