The term commonly refers to Bell's Poultry Seasoning, a combination of dried rosemary, oregano, sage, ginger, and marjoram, this seasoning mix has been around since 1867 when it was invented by Boston cook, William G. Bell. It's called poultry seasoning because among its famous uses is as a seasoning for stuffing.
Most supermarkets in New England and the Northeast carry this seasoning, but it may be harder to find in other parts of the country. Look for it next to other spice mixes like Old Bay. With a blue turkey on the box, it's pretty hard to miss. It's also available by mail order.
Store in a cool, dry place as you would other spices.