We can attempt to define it here, but any definition will be met with heavy resistance... as you can tell from the rifles being submitted.
Many of these rifles may be used for "stalking" but does that make them "stalking rifles?"
If this is a category of rifles it should have some parameters that are defined.
I will give it a shot, and then duck the shrapnel.
Stalking Rifle;
A "Stalking" rifle is a slender and wieldy bolt action rifle, of lighter weight and is generally (but not always, these days) possessing blued metal in a nicely grained walnut stock. It wears a short to medium length light Sporter contour barrel, usually with functional iron sights, but may be scoped with a low-power optic. It is most commonly chambered in an accepted medium/large game cartridge from 6mm to .375 cal.
That is sure to ruffle some feathers, but I would suggest that there is a difference between a "Stalking Rifle" and a rifle used while stalking.