DoS and Hacking

Based on the top rated answer to this question:

Is showing your IP address in the URL a bad practice?

I'm curious to know if a DoS attack is consider 'hacking' or simply an 'attack.'

I think definitions are critical to a field (I would bet most of us differ on what qualifies as an APT..) so my question is:

Is DoS'ing a site considered hacking it? Why or why not?