The waveform function underlying everything; the Universe's brain waves; a message from before time that we are not equipped to read. People call the Signal all sorts of strange things. Whatever you want to say its origin is, it nevertheless exists and can be heard by a sufficiently clear human mind, far away from the Noise generated by other life. A familial predisposition toward sensitivity has been observed, but even the geneticists of the Third Wave planets are divided over how it is passed along.
Everything generates a Signal, technically, or a smaller version of it – human minds in particular put off multiple, scattered mini-Signals, which reinforce and interfere with each other and are referred to as Noise.
A note on usage: when the definite article is used without reference to a specified object (simply the Signal), or someone references the Great Signal, it generally refers to the massive, echoing Signal that is the sound of the Universe. When the indefinite article (a Signal) or a possessive (the ship's Signal) is used, that of an object or phenomenon is being referenced. Not hard and fast rules of usage, but usually correct.
Changing the Signal Edit
Manipulation of the Signal is extremely difficult, but can be done. Doing so causes a quite literal change in reality around the manipulator, giving rise to nicknames like "magician" or "wizard" for people who can affect it. Most effects that can be produced are minor, with small expressions of heat, kinetics, gravity and electromagnetism being among the simplest effects. Manipulation of the nuclear fundamental forces is technically possible, but usually even one's own Noise is enough to block out attempts by anyone other than a grandmaster.
From remote monasteries on asteroid bases to hyper-sanitised deep space stations, over the course of several centuries there have been many different locations devoted to the study of the Signal by many different people. The most enduring group to have studied it have no formal name they give themselves, but are often known as Sages to those aware of their obscure, almost cult-like line.