The Orbital Defence Initiative (abbreviated ODIN) is an orbital satellite weapon; a very powerful remnant of the Sixty Minute War and a major feature of the third and fourth books in the series, Infernal Devices and A Darkling Plain.
It was built as part of the arms race between the American Empire and Greater China. It and MEDUSA are the only superweapons known to have survived until the events of the series, although there are several references to other orbital superweapons (Diamond Bat, Jinju 14, and the Nine Sisters for example). ODIN is more powerful than MEDUSA and is able to hit almost any target on the surface of the earth - MEDUSA was a ground-based energy weapon while ODIN is based on a satellite. ODIN is supposedly an American satellite as the code for controlling the satellite came off an American submarine. The other orbital weapons are hinted to have broken up over time and fallen from the sky.
ODIN is an energy weapon which apparently converts the energy of a small nuclear weapon into a directed beam of incinerating energy, based on the Strategic Defense Initiative of the 1980s. This has the power to exterminate cities (both traction and static) and provoke volcanic eruptions. Its beam can be seen for very long distances and seems to interfere with the mechanical minds of Stalkers. Only Shrike's Old-Tech Stalker brain has the mettle to withstand this, although he goes into a fit-like state and it is hinted he is saved by Dr Oenone Zero. Anna Fang, whether because of her connection to ODIN or her particular mix of Old-Tech and modern Stalker brain, is unaffected. Other Stalkers just lose all power.
The Tin Book, copied from a US Military document recovered by the refugees of the original Anchorage from a submarine, is a codebook for controlling ODIN. It is stolen by the Lost Boys (Mortal Engines) and, later, Brighton. It then falls into the hands of the Stalker Fang, who memorises its contents and then leaves it to be destroyed on Cloud 9 when she is attacked by Shrike. It is then destroyed in a fire; however, when Fang is rebuilt by Fishcake, her Stalker alter-ego travels towards Batmunkh Gompa to firstly steal parts for a transmitter with the power to reach ODIN and then onto Anna Fang's living residence, where there is enough Old-Tech to finish the transmitter. The journey is delayed by Anna Fang's confused and caring personality coming to the fore. But at Batmunkh Gompa the Stalker takes control until the final scene (apart from a brief period of a few seconds as Dr. Popjoy examines her mechanical brain).
When ODIN is fired, confusion ensues as both the Green Storm and Traction Cities are targeted. Both sides try to find the transmitter, leading to the Storm's assault of London, but it is Tom, Hester, Shrike and Pennyroyal who find Fang. As Fang prepares to target all the volcanoes on the earth and so destroy humanity, a final confrontation from Tom brings Anna to the fore once more and she orders ODIN to turn its beam weapon upon itself, destroying it completely.
As well as its immensely powerful weaponry, ODIN appears to show signs of intelligence. When it is awakened, it queries its new position and briefly searches for its old masters, it also notes the vast difference in geography since its last awakening. It can also zoom in to an individual's face on the Earth and, although the picture is grainy, it is still impressive. It can change its orbit when directed to target all over the globe. This, as well as the Stalker minds found among old-tech (and Shrike) seems to prove that what we loosely call robots had, by the time of the Sixty Minute War, achieved sentience.
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