Aquila

The Aquila is a two headed eagle, and represents the entire Imperium of Mankind. During the times before the Horus Heresy, the two-headed-eagle was blind-folded on the left side, and the other side had eyes. The blind side of the symbol represented mankind looking back into its past at the lessons of its history and traditions, while the sighted eagle was looking into the future and the hope of the better life that the Emperor would create for humanity. After the Heresy, as the Imperial Cult took hold over the minds of men, the Aquila's meaning changed to represent the political union of the Imperium of Man that had been founded on Earth with the Adeptus Mechanicus of Mars.1

Sign of the Aquila
The Sign of the Aquila is a hand gesture used to show one's allegiance to the Imperium of Man. There is some debate as to how the gesture is made: the Sign of the Aquila has its user place their hands flat on their chest with their thumbs interlocked. This forms the image of the Imperial Eagle on the chest with the thumbs forming the head and the straight fingers symbolizing its wings.2

Equipment
The Aquila is also displayed on Space Marine Power Armour, and stamped on weapons produced for the Imperium at its Adeptus Mechanicus Forge Worlds. Bolters, lasguns, tanks, and every other piece of Imperial military equipment oftentimes displays the Aquila painted or stamped on them. The Aquila is also stamped on signet rings belonging to those serving in an Imperial office, in the military or in another arm of the Imperial government like the Administratum.