Any watch can be considered a tool watch in the sense that their primary purpose is to tell time. I'm a lawyer - the only "tool watch" I need is a three hand dress watch, date optional (when it comes down to it, I don't even need that, as I can always get the time from my phone, my desk computer, my office phone display, or a wall clock).
That said, while I own a nice dress watch, I find it kind of boring. I grew up watching James Bond movies and The Undersea World of Jacques Cousteau, so I find myself gravitating toward sport watches. I'd much rather wear my SD, Speedy, or Doxa - they just look cooler to me. I'm under no illusions that I'm using them as "tool watches" however, other than to tell time (somtimes I use my Speedy Pro to time my son's math homework

).
As others pointed out, many seem to use the term to denote a rugged looking watch with a matte finish. I hear the term associated a lot with the Tudor Pelagos, which is obviously very similar to a Rolex Sub. It clearly gets the "tool" moniker because of the way it looks.