the same city; the city in question
Used in news reports and formal contexts to refer back to a previously mentioned city. Often appears in phrases like 同市内 (within the same city) or 同市出身 (from the same city).
The population of the city is about 500,000.
He is a writer from the same city.
He is an actor from the city (mentioned above).
A fire broke out within the city.
同市 anaphorically refers to a city just mentioned, while 同県 refers to a prefecture, a broader administrative unit.
同町 refers back to a previously mentioned town, while 同市 refers back to a previously mentioned city, so pick the one matching the municipality type.
同国 points back to a country, while 同市 points back to a city, a much smaller unit, so they aren't interchangeable.
Compound of 同 (same) and 市 (city). A straightforward Sino-Japanese formation used as an anaphoric reference in formal writing.