Metropolis / (administrative) 'to' — a prefectural-level designation; often refers specifically to Tokyo Metropolis.
『都』はしばしば東京を表す言葉として使われる。
The term 'to' is often used to refer to Tokyo.
東京都（とうきょうと）は日本最大の都である。
Tokyo Metropolis is Japan's largest 'to'.
A counter for cities/towns: 'to' can be used as a counting unit for certain administrative areas (e.g., in expressions like 'one to, two fu, three ken').
一都二府三県という表現では、最初の『都』が東京を示す。
In the expression 'one to, two fu, three ken', the initial 'to' indicates Tokyo.
Capital: the 'to' meaning capital or metropolis (akin to みやこ), historically the political and cultural center of a country.
平安時代、京都は日本の主要な都（みやこ）だった。
In the Heian period, Kyoto was Japan's main capital (miyako).
古くから都として栄えた街には歴史的建造物が多い。
Cities that flourished as capitals historically have many historic buildings.