Britannia was the name given to Britain by the Romans. Since then it has been shortened to Britain.
But that was not the only use of Britannia. She is seen as a mythical figure on many buildings (like the Liver Building in Liverpool, shown here) and also on some old penny coins. Why is this?
Queen Elizabeth I had promoted Britannia as a woman of power like herself. In 1672 Britannia made her first appearance on English coins, and as England became united with Scotland and then Ireland, so Britannia became an important symbol.
As the British Empire grew in the 18th century, so people began to look for something to symbolise Britain. This is how Britannia came to be viewed as the image of Britain.
British power depended on her strong navy, so gradually Britannia became connected to the seas (Hence the patriotic son 'Rule Britannia'). By now Britannia was shown more or less as a Greek or Roman goddess. The connection with the sea was reinforced by adding the god of the sea Poseidon's three-pronged trident and putting tall-masted ships in the background. The shield by her side was also a Greek hoplite shield, representing power. It was a handy place to add the Union Flag. Then at her feet was the British Lion, an animal which had been found on the arms of England, Scotland and the Prince of Wales for centuries.
Britannia becomes a symbol particularly in times of war.