Item from Mass Effect by BioWare
The galaxy-spanning transit network that makes interstellar civilization possible — and the Reapers' first infrastructure, built to concentrate and control spacefaring species.
The mass relays are supposedly Prothean technology. Every spacefaring civilization discovered them after their industrial age, used them to expand into the galaxy, and built civilization around access to the network. The network enables the Citadel Council. It enables interstellar trade. It enabled the First Contact War and everything after it. The relays are Reaper-built. This is one of ME1's revelations: the technology that defines galactic civilization was placed there deliberately, as a forcing function, to concentrate species in predictable locations before each harvest. The mass relay network is the Reapers' most elegant weapon because it is also the galaxy's most valuable infrastructure. Destroying a relay kills everyone in the system. Shepard knows this. The choice in the Arrival DLC makes it load-bearing.
Individual mass relays are enormous ring-shaped structures of Reaper-derived metal, floating at specific gravitational nodes throughout the galaxy. They are far larger than any ship that uses them. Each relay contains an eezo core that generates a mass effect field allowing ships to accelerate to FTL speeds along predetermined lanes. The effect is instantaneous from the ship's reference frame.
Also known as: Mass Relays, Mass Relay, Relay Network