The term "hacker" is usually associated with technology and, sometimes, more specifically, with programming, but I don't think that captures the essence of what a hacker is. A hacker is just someone who likes to tinker with things.

The term "things" is intentionally ambiguous because it varies greatly from person to person and includes much more than just technology. Indeed, while hackers usually enjoy playing around with code or electronics, it isn't uncommon for hackers to be well read in completely unrelated subjects such as poetry or martial arts. Such behavior makes sense in light of an inherent playfulness.

It also explains, to a degree at least, the connection to startups. While I'm admittedly ignorant of much of the business world, it seems that startups would be the most "playful" (in sense that startups provide the most room for trial and error). Larger or more established ventures are sort of "on rails." They already have momentum and don't want to jeopardize it so they'll naturally be more hesitant to try new business strategies, especially ones without a proven track record.

Essentially, what defines a hacker is the desire to manipulate ideas and concepts, i.e., an innate intellectual curiosity. Most things on HN fall under that umbrella.