Definition of Eureka flag in US English:

Eureka flag

noun

Australian
  • A blue flag bearing a white cross with a white star at the end of each arm and in the centre (in a stylized depiction of the constellation Crux); = "Southern Cross".

Origin

Late 19th century; earliest use found in Ballarat Courier. From the name of the Eureka Stockade in Ballarat, Victoria, site of the 1854 rebellion during which the flag was first used.