What does "city state" mean?

A sovereign city, as in Ancient Greece, often part of a league of such cities.

What is a synonym for "city state"?

Common synonyms include: ally, archduchy, archdukedom, body politic, bremen, buffer state.

noun

noun ·Uncommon ·Advanced level

Noun

1
A sovereign city, as in Ancient Greece, often part of a league of such cities.
"Singapore is the world's most populous city state and the only island city-state in the world."
2
a state consisting of a sovereign city wordnet

"Singapore is the world's most populous city state and the only island city-state in the world."

From city + state, a calque of German Stadtstaat.