The history and etymology of place names.
This tag should be used when discussing the nomenclature of places, peoples, events, etc.
If the question is purely linguistic, then it is probably out of scope for H:SE; judgement is required. The question should address toponymy in some historical context.
Questions about what a given place was called at a given time are important to the study of history, and would be in scope. (Bull Run vs Manassas is essential if you want to understand the Civil war; Bunker Hill vs Breeds Hill is trivial, but may assist in interpreting US Revolutionary War reports).
Questions about how a placename changes may be important to history or may illustrate history (Constantinople -> Istambul, Leningrad->Stalingrad, etc.)
Simple questions about why X is called Y with no historical context, are probably out of scope ("What did Detroit mean in the original Amerindian language?" is probably out of scope; "What is the etymology of Pennsylvania?" might be in scope. The first is an accident of history - the second is a choice that illustrates the plan of a historical figure.)