On Australia's flag, what is the large star below the Union Jack called?

On Australia’s flag, the large star below the Union Jack is called the Commonwealth Star.

It sits in the lower hoist quarter, the section nearest the flagpole. The star symbolizes Australia’s federation: its original six points represented the six colonies that became Australia’s states on 1 January 1901.

The star initially had six points. A seventh was added after Papua became an Australian territory, and that point came to represent Australia’s territories and any future states. This is why the Commonwealth Star is also sometimes called the Federation Star.

It is easy to confuse the Commonwealth Star with the Southern Cross, the group of five stars on the flag’s fly side. The Commonwealth Star is a single emblem beneath the Union Jack; the Southern Cross is the constellation depicted on the opposite half of the flag.