- "Since before your sun burned hot in space, I have awaited a question."
- — The Guardian
The device known as the Guardian of Forever is an artifact of an ancient alien race (most likely the Guardians), which functions as a time portal, located on the planet Gateway. The Guardian is apparently sentient; it can speak and responds to the questions and actions of individuals who come in contact with it. It generates immense ripples in time that manifest themselves as spatial disturbances in the surrounding region (perhaps shielding its planet and star system from changes in the timeline). (TOS: "The City on the Edge of Forever")
Apparently an inert formation of a quasi-metallic substance, the Guardian is neither machine nor being, describing itself as "my own beginning, and my own ending." It has the ability to create portals to other times, which can be watched or traveled through.