Practically the same thing.
Strictly speaking High Definition Television (HDTV) is not the same thing as High Definition (HD). However, the difference is insignificant so you will hear the two terms interchanged in everyday use.
HDTV is a subset of Digital Television (DTV). This new standard is mandated to replace the old NTSC analog broadcast in the near future. DTV specification created by ATSC defines a multitude of different frame sizes and frame rates for digital television broadcasts. Several of those are HDTV standards.
HDTV implies “televised” nature of high definition images. It is television delivered to consumers via cable, satellite or terrestrial broadcasts.
HD is high definition in a broader sense. During production and post production high definition video is not television yet. HD becomes HDTV once it is delivered to your home.