The eagle plays an important part in heraldry in almost every part of the globe. Its earliest rise to popularity, however, was in Germany, where, after it became the emblem of the empire, it was adopted by some of the princes and many of the nobles. A double-headed eagle is also the emblem of Russia and Austria. On the roll of Henry III the eagle appears but twice, but in the roll of Edward II there are forty-three examples of it. Nobles of the Holy Roman Empire place their shields on the breast of an eagle, examples of which may be seen in the arms of the Duke of Marlborough , the Earl of Denbigh and Lord Arundel of Wardour.
The imperial eagle is invariably represented as two-headed, the origin of which is obscure. Charlemagne is said to have used it to signify that in his hands was the government of both the Roman and the German empires.
The eagle is generally borne displayed; that is, upright, breast to the front, and legs, tail and wings expanded (commonly called a "spread eagle").
The Bald Eagle, adopted as the national emblem of the United States, is a large and powerful bird, with a far greater spread of wing than the European white-tailed species.

This definition is in context to Heraldry. See more contextual defintions for Eagle.