HD = High Definition. It's a term to describe high quality video. You often see it quoted as 720p, 1080i, or 1080p. But the term is SUBJECTIVE.
http://en.wikipedia.org/wiki/High-definition_video
Many "Fandroids" claim that the iPhone is not HD because it doesn't have 720 pixels in resolution.
The "not HD" claim is NOT TRUE.
Remember, Apple uses the "Retina Display" marketing term, which really describes ppi, or pixels per inch. The iPhone has 326ppi. The reason they use the word retina is because your retina in your eye can't discern the pixels when holding the phone away from your face. This is critical, because it means that Apple DOESN'T NEED to go to 720p to get an HD quality image.... you already have an HD screen as much as it's ever going to be. Your eyes can't see any more than that.
The only way to get the 720p that is discernible that Fandroids talk about is by making the screen even bigger, beyond the 4''. But they are just quibbling about pixels... not actually what HD actually means.
Remember, 720p and 1080p are terms used to describe TV viewing from long distances... that is why they were made up. The farther away you sit from a TV, the less resolution you need on your TV. That is why new 4K TV's need to be at least 75'' for consumers to notice a different... any smaller and you wouldn't even be able to see the 4K resolution!
The term HD is really irrelevant when it comes to modern mobile devices, because most new phones have a high enough ppi. If anyone tells you otherwise from what I just said, they are uneducated.
Ethan