A signature dish is a recipe that identifies an individual chef or restaurant.
Ideally it should be unique and allow an informed gastronome to name the chef in a blind tasting.
In a weaker sense, a signature dish may become associated with an individual restaurant, particularly if the chef who created it is no longer with the establishment.
It can also be used to refer to a culinary region, in which case its meaning may be the equivalent of "national dish".
At its weakest, the term can simply mean "chef's specials".