Faster Than Light
Specifically in reference to Faster Than Light Propulsion, this article focuses on how the FTL drives work and how speeds are determined. Under simplest terms, a ship is able to travel faster than light by moving itself partially out of normal space.
FTL Factor determines how fast a ship is moving. This is a relative speed, as ships traveling at FTL never stop accelerating.
Gravitational Distortion Issue
Most FTL drives that keep the ship in normal space, such as Python and Slipstream drives, as subject to gravitational distortion. The gravity fields from stars and planets affect the drive field, and result in unpredictable bursts of acceleration. This precluded the use of FTL within most solar systems(though, jumps can be made from the outer solar system if there are no large planets in the way), and causes significant navigational concerns for FTL courses. Generally speaking, a course is plotted that does not pass near any intense gravitational fields.