I believe plenty of dvd’s are HD, high definition films as seen on an HD tv, blue ray being a further hd enhanced format.
DVDs cannot be HD. They are limited to 720 X 480 resolution, while Blu Ray is 1920 X 1080 resolution.
Many good DVD players make DVDs look remarkably good on a quality HD display. Blu Ray does look much better than either DVD or over the air HD on a big HDTV.
No DVDs are HD. The lowest HD resolution is 1280x720 (720p). The max DVD resolution in NTSC is 720x480, which is closer to wide-screen 480p SD.