The flag of Australia is a defaced

Defacement (flag)

Defacement is a term used in heraldry and vexillology to refer to the addition of a symbol or charge to another flag. For example, the Australian flag is the British Blue Ensign defaced with the Southern Cross in the fly and the Commonwealth Star in the lower hoist quarter, beneath the Union...

 Blue Ensign

Blue Ensign

The Blue Ensign is a flag, one of several British ensigns, used by certain organisations or territories associated with the United Kingdom. It is used either plain, or defaced with a badge or other emblem....

: a blue field with the Union Flag

Union Flag

The Union Flag, also known as the Union Jack, is the flag of the United Kingdom. It retains an official or semi-official status in some Commonwealth Realms; for example, it is known as the Royal Union Flag in Canada. It is also used as an official flag in some of the smaller British overseas...

 in the canton (upper hoist quarter), and a large white seven-pointed star known as the Commonwealth Star

Commonwealth Star

The Commonwealth Star is a seven-pointed star symbolising the Federation of Australia which came into force on 1 January 1901....

 in the lower hoist quarter. The fly contains a representation of the Southern Cross

Crux

Crux is the smallest of the 88 modern constellations, but is one of the most distinctive. Its name is Latin for cross, and it is dominated by a cross-shaped asterism that is commonly known as the Southern Cross.-Visibility:...

 constellation, made up of five white stars – one small five-pointed star and four, larger, seven-pointed stars.

The flag's original design (with a six-pointed Commonwealth Star) was chosen in 1901 from entries in a worldwide competition held following Federation

Federation of Australia

The Federation of Australia was the process by which the six separate British self-governing colonies of New South Wales, Queensland, South Australia, Tasmania, Victoria and Western Australia formed one nation...

, and was first flown in Melbourne

Melbourne

Melbourne is the capital and most populous city in the state of Victoria, and the second most populous city in Australia. The Melbourne City Centre is the hub of the greater metropolitan area and the Census statistical division—of which "Melbourne" is the common name. As of June 2009, the greater...

 on 3 September 1901; this date has been proclaimed as Australian National Flag Day.