A borer is a type of slipspace drive created and used by the Covenant and its remnants. It is a spacecraft propulsion system capable of making transitions to and from slipstream space and by extension allowing faster-than-light interstellar travel.
Being more technologically advanced than humanity, the Covenant have numerous advantages in slipspace propulsion systems. Whilst the human Shaw-Fujikawa engine is said to “punch” a hole between realms using brute force, Covenant engines instead take a small rupture and delicately enlarge it with surgical precision. This allows the latter to execute far more accurate slips. Covenant slipspace drives are often referred to as "jump drives". In addition to their more powerful thruster engines, it has been theorized by the UNSC that Covenant drives generate several "microjumps" within a single slipspace transition to measure the dilation involved in a jump, allowing them to reach their destinations faster.
Covenant drives are generally more flexible and powerful than those of humans. They have thrice been seen to execute in-atmosphere slipspace transitions (although the first time the drive in question was controlled by a human AI). In addition, Covenant drives can execute successful slips even if underpowered.
Covenant slipspace drives can also be refitted to human ships, as seen during Operation: FIRST STRIKE, where the UNSC fitted Ascendant Justice's slipspace drive to the UNSC Gettysburg to escape Unyielding Hierophant and reach Earth faster.