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).
同市の人口は約50万人です。
The population of the city is about 500,000.
He is an actor from the city (mentioned above).
彼は同市出身の作家だ。
He is a writer from the same city.
同市内で火災が発生した。
A fire broke out within the city.
同市 anaphorically refers to a city already mentioned, whereas 同国 does the same for a country.
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.