The free online dictionary defines touring as:
"Travel, as on a bicycle or on skis, for pleasure rather than competition."

So if it is for "pleasure rather than competition" then by at least one definition it is touring. By that measure, you can feel free to call pretty much any pleasure ride a tour.

That said I think most of the bike tourists mean overnight or longer trips when they say touring. Personally that is the bar above which I consider my trips tours.

The description of this forum reads:
"Have a dream to ride a bike across your state, across the country, or around the world? Self-contained or fully supported? Trade ideas, adventures, and more in our bicycle touring forum."

That implies a bit higher bar, so I would think that discussion of day riding fits better in the general forum.

I have never heard of anyone having a particular mileage qualifier, but I wouldn't really consider my trips bike tours unless the riding was at least a somewhat prominent part of the trip.

Bottom line... Call anything you like a tour, but expect to get a little flak here if posting about day rides.