It probably has multiple definitions anyway, the one I've got ready to bake is the more modern version, also sometimes called shepherds pie or potato pie.
Savoury meat and gravy filling (plant based in my case) in a pastry pie crust, with mashed potato instead of a pastry lid on top.
More traditional shepherds pie doesn't have the crust, leftover roast meat and vegetables in gravy with mashed potato on top, baked and cut into individual serves haha