Any
group, especially a
nonprofit, that has as its purpose to
investigate things that could be
dangerous to the
public.
I've heard the term watchdog used mostly to describe progressive groups such as Fairness and Accuracy in Reporting, but there are plenty of conservative watchdog groups including the NRA and politically neutral groups like Consumers Union.
Of course, some watchdog groups are really astroturf groups set up as a front for powerful interests. My philosophy is: the more canaries in this mine, the better.