It is very difficult to try to hold “À la Diane” to too tight a definition, other than that the meat in a dish with this name will be game.
The meat is often, but by no means necessarily, venison. The meat is often sautéed and served with “sauce diane”, chestnut purée and croutons (small toasts) spread with puréed game on them.
It can also mean just a meat purée (forcemeat) made from game. A classic application of the purée is spreading it on soft-boiled eggs on toast with sauce salmis, mushrooms, or with a cream soup flavoured with port.