The symbol for Eagle is often used on for example coat of arms and flags. The sign, in this example, is used in the US as great seal for official documents and pasports.
This symbol stands among others for the Roman god Pluto (god of the underground) and the planet Pluto (named after the roman god). The symbol can be explained as the initials of it's predictor Percival Lowell or the first 2 letters of Pluto. Accross each other they form this sign.