Tomorrow, Apple is probably announcing a new iPad with a crazy 2048 x 1536 display that it might call the iPad HD, not the iPad 3. But HD is possibly the most meaningless term in technology.
That's because it probably has the most meanings--sure you've heard of an HDTV, but what about an HD glove? (It exists.) Some alternative definitions below.
Basically any HDTV today
What HD means: a 1280x720 (720p) or 1920x1080 (1080p) display
Apple Cinema Display HD
What HD Means: a 23-inch, 1920x1200 display
Samsung Galaxy S II Skyrocket HD
What HD means: a 4.65-inch, 1280 x 720 HD Super AMOLED display