Time Travel Theories

Variable Thread (VT): Traveling through time alters the timeline. If you go to the past, you can alter the present.

Fixed Thread (FT): Time is fixed, and traveling through time is like looping the thread back on itself. "Changes" you make to the past already happened, and traveling back is more like changing perspectives. This method of travel has to do with accomplishing more in the same amount of time than actually changing anything.

Multiverse: Rather than actually traveling through time, you travel laterally to similar - but slightly older or younger - parallel universes. 

Other Terms

Traveler: The person, device, information, or consciousness traveling through time. Sometimes also referred to as the "subject."

Paradox: Altering the timeline in such a way as to create a contradiction of events in the timeline.

Origin: The time from which the subject travels.

Destination: The time to which the subject travels.

Outsider: The traveler in Variable Thread. Once they enter the timeline, they alter it such that they no longer belong in it.

Causal Loop: A self-manifesting concept in which the something (be it information, an object, or even a person) sets in motion the events that result in its own creation.