definition
strict definition
borough
A town or district with its own charter, at a tier that differs from country to country.
A borough is a place holding a charter of self-government. What the charter is worth differs widely: a New York City borough is one of five large divisions of the city, a Pennsylvania borough is a small municipality on the same footing as a town, and an Alaskan borough is what other states call a county.
The word therefore says nothing about size or rank on its own, and the tier has to come from the country's hierarchy. A London borough governs more people than many cities.
Where a country's own register calls the unit a borough, keep the term rather than translating it into a more familiar one.