Commonwealth is a traditional English term for a political community founded for the common good. Historically it has sometimes been synonymous with "republic".
The English noun "commonwealth" in the sense meaning "public welfare; general good or advantage" dates from the 15th century. The original phrase "the common-wealth" or "the common weal" (echoed in the modern synonym "public weal") comes from the old meaning of "wealth", which is "well-being", and is itself a loose translation of the Latin res publica (republic). The term literally meant "common well-being". In the 17th century the definition of "commonwealth" expanded from its original sense of "public welfare" or "commonweal" to mean "a state in which the supreme power is vested in the people; a republic or democratic state". "Better things were done, and better managed ... under a Commonwealth than under a King." Pepys, Diary (1667)
Three countries – Australia, the Bahamas, and Dominica – have the official title "Commonwealth", as do four US states and two US territories. More recently, the term has been used for fraternal associations of some sovereign nations, most notably the Commonwealth of Nations, an association primarily of former members of the British Empire, which is often referred to as simply "the Commonwealth".