The Aquila (also known as Imperial Aquila) refers to a two-headed eagle that symbolizes the Pax Imperialis, the oath that binds together all Imperial servants under the banner of the Emperor of Mankind. 
It is a universal symbol within the Imperium and more than anything else represents the dominion of the Emperor. The double-headed eagle also represents the union of the Adeptus Mechanicus of Mars and the Emperor or between Mars and Terra.
imperial aquila