"There is evidence to suggest that the geometry drive is a device that has been specifically tailored for ease of use and construction, even for a somewhat low-tech civilisation. An open-source faster than light device, if you will." -- quote attributed to Rani Spengler.

The canonical presentation of a geometry drive as a blue, crystalline hypercube.

Some purists would argue that calling the geometry drive a faster-than-light device is misleading: by definition, the geometry drive doesn't allow a ship to concretely travel faster than light. There is a grain of truth to this assertion. The geometry drive isn't an engine. It is an imprint upon the world, a physical object whose presence allows for nigh-instantaneous translation of an object from a location in three-dimensional space to another location. This is the what of the geometry drive. The how remains a mystery, even a century and a half after its discovery. The general assumption is that the geometry drive somehow folds or alters three and four-dimensional space to allow for teleportation with unlimited range, perhaps creating the equivalent of a temporary wormhole. The actual process through which this teleportation is achieved remains unknown. As a geometry drive requires little power to operate, it is speculated that the mere presence of a drive, once fed translation data, is sufficient to achieved the desired effect. These unknowns remain because the geometry drive is not an invention, it is a discovery.

Indeed, the geometry drive was found by one Rani Spengler, a century and a half ago, in the wreck of an ancient solar sail cruising at the edge of the solar system. Though the work of a more advanced society, it was easy to reverse-engineer even without an understanding of its physical principles -- in that regard humankind's relationship with the geometry drive can be considered a form of elaborate cargo cult. The leading theory is that the drive isn't an alien artefact, but was rather sent into the past by future humans, who themselves received it from our present: stuck in closed time loop, the geometry drive has no inventor, and it not subject to the relation between cause and effect. 

Punched paper cards containing basic translation data for a freighter drive*.

The physical appearance of the geometry drive sets it quite apart from modern space technology. It presents itself as a four-dimensional cube, or tesseract, made of a crystalline compound harvested on the flowers of pseudonigella stellaris. Drives are stored in canisters which are to be installed as close as possible to the ship's centre of mass -- this is not a hard requirement but greatly facilitates computation. Geometry drives are inert and transparent when not connected to a power source; when under power, they glow in white-blue, sometimes yellow or red depending on the origin of the tesseract. A geometry drive is often the safest part of a spaceship: as they are very low-energy objects, they softly shatter in case of heavy damage instead of shorting or exploding.

The teleportation caused by a geometry drive is called a translation. The term "jump" is sometimes used as well, albeit is incorrect. The main characteristics and limitations of geometry translations are called the ten observables. They are as follows:

• It is a true teleportation, not merely fast travel. As such, it can go "through" objects such as planets.
  
• Neither the end nor beginning can be in dense matter. A drive cannot reintegrate an object in anything denser than a planet's exosphere and cannot start a translation from there either. This is a fundamental limitation, sanctioned by a translation failure upon computation. It is assumed the drive has a means of "pinging" its surroundings and the destination before translating. As such, geometry drives cannot be used to launch ships into space, nor can they be employed to teleport objects or weapons inside a target.
  
• Complexity of navigation is exponentially proportional to distance. The economy of faster than light travel relies on computation capability; the complexity of the four-dimensional equations grows exponentially with distance. Modern navigation software cannot handle translations beyond fifty lightyears.
  
• Translations are relative. As all objects in the galaxy are in motion, identical trajectories must be recalculated in real-time.
  
• Momentum is conserved. Thus, when translating towards a destination, a ship needs to match its relative velocity.
  
• Energy is conserved. This mainly applies to potential gravitational energy, as explained here. Drives cannot be freely used in large gravitational wells, due to the necessary compensation of lost or gained potential energy being enough to melt or destroy a ship.
  
• Drives interdict each other. Translation is impossible (inbound or outbound) within a thousand kilometers of another geometry drive.
  
• Chained translations cannot result in a perpetual motion machine. The use of repeated translations to create a perpetual motion machine, or an infinite energy generator, are sanctioned by the self-destruction of the drive, presumably to preserve the state of the universe.
  
• Time travel is possible with caveats. Like all faster than light devices, the geometry drive allows for time travel. However, it seems to be following a rather arcane set or rules regarding this phenomenon, and disallows ships from crossing into their own light cone, often by self-destruction or random displacement. This shackle also exists to preserve the cohesion of the universe.
  
• The drive has a mind of its own. It is not uncommon for geometry drives to start acting strangely, to refuse to translate or to pause without warning. 

Credit: Marcin Wichary, Flickr picture / Hypercube: Mouagip under CC-BY-NC-SA 3.0.