The Nexus is the hive-mind-like network used by the Sen'kaal to telepathically communicate. It is believed to work via neutrino pulses. Those Sen'kaal (and humans) capable of sensing and using the Nexus are known as Nexus-sensitives or biotics.
Role in the Sen'kaal Dominarchy
Within the Dominarchy, the Nexus serves as a sort of "internet" through which Sen'kaal can communicate, commanders can relay orders, and information can be transferred. Many pieces of Sen'kaal technology can only be activated using the Nexus.
Role among Humans
After the end of the Human-Sen'kaal War (Contact '57), a number of UN soldiers began to report symptoms including hearing voices, feelings, and thoughts. The militaries of their respective countries dismissed this as battle fatigue, but, after USAF General Curtis Lemay checked himself into an Air Force hospital with these symptoms, the U.S. military launched an investigation. It was discovered that exposure to sordium, an element found in most Sen'kaal weaponry, caused a brain mutation enabling humans to use the Nexus. Human military commanders were quick to recognize the combat potential of these individuals, and most militaries established specialized 'academies' to train and hone the skills of these telepaths.