As sterlingice noted, a TD is a tropical cyclone with maximum sustained winds of less than 39 mph. A tropical cyclone is defined as:
A warm-core non-frontal synoptic-scale cyclone, originating over tropical or subtropical waters, with organized deep convection and a closed surface wind circulation about a well-defined
center. Once formed, a tropical cyclone is maintained by the extraction of heat energy from the ocean at high temperature and heat export at the low temperatures of the upper troposphere. In this they differ from
extratropical cyclones, which derive their energy from horizontal temperature contrasts in the atmosphere (baroclinic effects).