If you were to ask someone fifty years ago what the world would be like today, that person likely would've said that we'd all have robot butlers, flying trains, and personal spaceships. Well, none of that has happened yet, but we DO have these HD televisions!
What's the defintion of high definition? Well, it's a definition that's higher than a low definition or a standard definition, thus making it a high definition. There you have it: the definition of high definition!