High-Definition. Digital TV using the MPEG-2 compression with a resolution of 720 or 1080 lines, interlaced (i) or progressive (p). Also has a wider aspect ratio (1:78:1 or 16:9) than regular TV (1:33:1 or 4:3), and 5.1 channel Dolby Digital AC-3 surround sound.
Abbrevation for Hit Die/Hit Dice used in the Dungeons & Dragons role-playing game.
"You cause 4 + 1d4 HD of creatures to fall into a comatose slumber, beginning with those with the fewest HD. Creatures with 5 or more HD are unaffected."
(Quote from the description of the Sleep spell in the Dungeons & Dragons based computer game Neverwinter Nights 2.)