Overdrive is the faster-then-light drive used by starships to propel themselves through space.
Starships use dark matter to create two fluid's, one super hot called a "plasma" and one super cold called a "solution," which they then force together to create an overdrive bubble which bends the space around the starship allowing them to travel faster than light.
The process is highly inefficant, 15% of the power generated being used is considered an amazing achievement. It is often said that working with an Overdrive engine is more art than science, as two engines can be built to the same specification yet produce different results.
The process is also not infalliable, in some areas of space it works better than others, these are called "spaceways" and whilst other areas, so called "Wild Space" so allow travel at overdrive, it is often much slower and some areas shut overdrive engines down completely.
Overdrive is announced to others in terms of factors between one and a hundred. "Overdrive factor three" for example. However, due to the navigation problems with finding spaceways and the problem of efficiency people very rarely get above overdrive factor ten.
Now whilst the overdrive engine is a great tool for travelling faster than light, when a starship is travelling slower than light it is a tremendous disadvantage, as it acts a major anchor slowing the ship down. Starships have to combat this by having enormous sub-light engines to propel them along, but can often be outpaced by smaller craft with smaller engines who do not have an overdrive engine.